My 521 has late 620 lower control arm bushings installed in it, this allowed me to install late 620/early 720 lower ball joint control arms and the late 620 disc brake tension rods, with adjustable tie rods you likely can use 720 drop spindles.
The upper control arms are harder to deal with, but if done with care, you could make 2 sets of 720 upper control arms into one set(lengthening them likely 3/4 of an inch), then smoothing the outside, then adding 620 bump stop bottom pieces to the control arms to close the bottom of the arm(box it) where it was modified, then drill the upper control arm mount holes and thread them for 720 mount bolts.
This will give you a ball joint front end with 85/86 720 vented rotor disc brakes, if you wanted to use your 320 rims then you would have to do Charlie69s lower control arm mod, but this will be impossible to hide, but if you just use a stock spindle your 320 rims will work with a little grinding of the rivets inside the rim, or use 521/620 rims without mods.
You would still be using your stock 320 steering column this way and it would not look scabbed together, but anyone that knows their stuff would know this is way safer than what the truck came with stock, but it would not be stock anymore nor would it look stock even though you are not changing the frame nor welding anything onto it, there is a little more involved but this is basically what I did to my 521 work truck around 12/13 years ago and most on here have seen the loads I have hauled with that truck, sure I had issues, but I addressed the worst one above, and I am not suggesting you put power steering in yours which really caused me some issues that I do not have a fix for except to modify the frame.
Now I have heard some stories about 320s over there, one was that they wanted a collapsible steering column installed, so they are not against changing things for the better, but I have no idea what would be involved engineering wise to satisfy the inspectors.
Were the LCAs you saw for sale stock or were they drop arms?
Hagerty doesn't insure daily drivers, or semi daily drivers, they insure classic vehicles that are driven to car shows/events and infrequent Sunday/pleasure drives, and the DMV in my state issues collector plates under the same rules as the insurance companies, less than 3000 miles a year which I believe is in the Hagerty rules, car shows/events, and Sunday/pleasure drives, and with Hagerty you also have to show you own a vehicle less than 10 years old(the daily driver), or show you have had a classic daily driver insured with a normal insurance company with regular insurance for several years like I did to have them cover my Roadster, and they check as you found out.
I never use my insurance either, I don't want to be dropped or have the rates go up, Hagerty is cheap, I have 2 classics insured thru them for less than $300.00 a year, I pay way more than that for all my other insurances and I have a lot of insurance policies, I likely fork out $4000.00 a year towards all my insurances(business, home, vehicle, classic vehicle).
I like return lines because of the heat issue Mike just described, cool fuel in the lines.
I actually have a stock regulator on the back SU(or I did until I changed SUs), if it starts vapor locking I will have to transfer the regulator to the SUs on the engine now, but only have them issues in the summer when it is hot out, it is cool now.
I also have an electric fuel pump now(last few years), as sometimes when I parked the truck on a hill(front pointed up hill) and the engine stopped in just the right spot, the fuel would siphon back into the tank and I could not keep the engine running on the fuel in the bowls, it would die and would not start unless I used a screwdriver on the fuel pump arm to get the fuel back up from the tank, I believe the fuel pump had a small hole/crack in the diaphragm and it would not suck the fuel up from the tank when it was very hot out, but that little extra movement in the arm that it didn't get to while running was enough to pump the fuel out of the tank, it finally got so bad I converted over to an electric fuel pump.
For a 521 there would be at least two other wires to a top gear switch that would turn the retard points OFF on the dual points distributor. The 521 (some anyway) had a neutral gear switch. So there's 6 wires.
Daniel and wayno have 521s maybe they know.
I thought about the dual point distributor and then decided to just let someone that knew what they were talking about answer.
I have not had any stock 521 since the mid 90s, the only wires I connect are the reverse light wires
The problem with them larger alternators is that when tilted way out like that when the engine shifts the alternator hits the idler arm and eventually it busts the bottom alternator mount bolts, and it will only tilt towards the block a little bit before it hits the upper hose.
I believe the 1981 alternator mount off a 720 Z22 lowers the mount location and allows you to tilt the alternator more towards the block getting it away from the idler arm,
You don't have to have a return line, but my SUs had one so I used that hard line as the return line and I routed the other end of that hard line straight to the tank fitting for the emissions stuff bypassing the small emissions tank.
Wait a minute here, you switched to different headlights, are you sure the headlights are wired the same as the square headlights the 720 has, I found when I switched over to round headlights on my 720 I had to switch two wires, the headlight plug forms a "U" shape, the bottom of the "U" stays where it is, but the sides of the "U" have to be reversed, I made a pigtail from another headlight plug for both sides.
Do not add a ground to the 720 headlight circuit, it has no grounds on the headlight wiring side of the relay, if you add a ground everything goes bad and your battery could drain over night and then it will not start.
My 520 had a power brake booster in it from a 73/74 Datsun 620, and it barely fit with a stock carb, I had to use a Ford Courier brake master because the 620 brake master reservoirs would not clear the carb, when I changed over to dual SUs I had to make a change as the back carb was in the brake master, so I installed a 521 brake pedal assembly in the 520 and now it barely clears with thin pancake filters, when I figured this all out I put a power brake booster in my work truck, should have done it years ago.
My 520 truck is LHD, I have no clue what a RHD 520 has for clearance, in my head I see lots of room.
He also asked about disc brakes, but that depends on if he is going with ball joints or is keeping the kingpins.
I suspect that the black wire with the red dots is a ground, both mine look the same(black/red dots) with both connected to an eye electrical fitting, I suspect if you put a timing light on both sets of spark plug wires and have at least the black/red dot wire not grounded, one of the coils will not be working, this happened on my 85 720 and I suspect the same thing would happen on the 86 in my driveway right now.
The yellow wire should be tested the same way, if it is not grounded do both coils work?
If you look on your front coil you will likely see that black/red dot wire connected to it, the back coil has much larger wires.