I don't like editing, if I screw up I don't want to hide it, but I now wish I had not said anything.
The 4X4 leafs from a Datsun/Nissan 4X4 were a one and a half inch drop on my 520, but my 520 is on a 1986 720 2wd frame, they were bolt in for the most part except for the bushings, and they are usually around $100.00 around here or cheaper on a holiday(half price), you know what your getting for the most part when you go this route.
The first link is for 4X4 leafs that the description seems to be correct, it has 2 arched leafs and 2 overload leafs, that is what the stock Datsun/Nissan 4X4s have, so I expect that you would get the one and a half inch drop that a OEM Datsun/Nissan 4X4 leaf would have, maybe a little less since the parts are new.
The Belltech lowering leafs with the 4 inch drop from carparts is likely for a stock 2wd 720, I don't know how much these leafs would drop your 520 compared to your 520 leafs, I suspect it is close to a 4 inch drop on a 720 2wd.
I did some measuring as I have all the leafs in my back yard we are talking about here except the belltech drop leafs, I did some un-sprung measuring,
the 4X4 720 leaf with a straight edge sitting on top of the leaf eyes measuring down to the mount surface of the leaf was 5 1/8th inches
the 720 2wd leaf measured the same was 7 3/4s inches
the 520/521 leaf measured the same was 6 1/8th inches
I actually have a set of 620 mobile home leafs also, they measure out 3 7/8ths inches
These are used leaf springs with no weight on them, if one was to take off 4 inches off of all these measurement except for the 720 4X4 leaf set which is a known one and one half inch drop it would measure out to,
4X4 720 leaf known drop is 1 1/2 inches with a 5 1/8th inch arch, these leafs are firmer than the 2wd leafs and in my opinion work the best with drop blocks.
2wd 720 leaf minis 4 inches works out to 3 3/4 inch arch, it will sag more than the 4X4 leafs as they are firmer than the 2wd leafs.
2wd 520/521 leaf minis 4 inches works out to 2 1/8th inch arch, but I believe that this leaf will sag the least under weight as it has more arched leafs(5 leafs).
If the spring rate is the same as stock OEM, I expect that the 720 2wd belltech lowering leafs will drop your truck the most, but you will likely be resting on the bump stops, they can be cut or removed but as these leafs are 2wd I suspect you will bottom out a lot unless they have stiffened them up.
The 4X4 leafs used with drop blocks can be dialed in to where you bottom out the least using the drop blocks that get you the most drop without hitting the frame.
Taking leafs out of your 520 leaf pack will lower the truck, but it will bottom out easier, if you are a dump the clutch type driver the axle will twist/flex the leafs(axle wrap), with a one piece drive line there is the chance of making the drive line wobble when one slams 2nd/3rd gear and the rear of the transmission case could come apart/destroy itself because the drive line gets pulled out of the rear of the transmission enough from axle wrap to have it wobble/get out of balance, this has happened to people on this forum.
Those with 2 piece drive lines talk about the axle twisting and hitting stuff on the under side of the vehicle when lowered, also when you burn out axle wrap can make the rear axle hop(wheel hop) which is why traction bars were invented for muscle cars.
In my opinion Datsun/Nissan 720 4X4 leafs with drop blocks are the way to go, you will get the best driver friendly result, but it is my opinion.
Using 4 inch drop blocks on your stock leafs will also get you low, but 4 inches is the ragged edge, as getting a flat in the rear could have the lower shock mount drag on the ground.
I went to the extreme to lower my 520, I have 4X4 leafs, 2 inch drop blocks and a wedge to correct drive line angle, I raised the carrier cross member, raised the front leaf mount, used the early Datsun Roadster rear leaf mount, I used early 510 wagon lower shock/leaf mounts, and I notched the frame, I re-indexed the front torsion bars and made front drop arms, it got me real low, so low I had to raise it to get in my driveway, I also had to re-route my exhaust above the cross members, it was a lot of work to do all this, but it is safe as I can get a flat and not drag anything on all 4 corners.
Here is my 520 build thread that has most these mods.