Just noticed, no doors on the cab. I am not saying it is the only way, but I was taught to fit the doors first, because they have to fit the cab, and seal. Then fit the hood, because it has to clear the cowl, and finally, the fenders.
Fit the doors by removing the catch, on the rear door post, and adjust until a dollar bill has a light drag all the way around the weatherstrip, on the top, bottom, front and back of the door. Match the back of the door with the style line on the cab, and when that is good, put the door catch back on, and adjust it so it holds the door snug against the weather strip, without too much pressure. Too much pressure, and it is hard for the lever on the door handle to actuate the latch mechanism, and release it, and the door handle breaks.
Then install the hood. Match the gap at the back of the hood, and then put the hood catch on. The 521 front hood catch can adjust the height of the hood a little. But note, there are four rubber bumpers the 521 hood. The two rear ones center the hood between the fenders. The two front ones support the hood height, and center the hood too. These bumpers are mounted in the fenders.
Now the fenders. Check the trapped nut on the front lower corner of the fender, on the tab that slides just behind the parking lights, and takes a bolt going forward, from under the fender, throught the core support. Usually chasing the nut witha 1/4-28 tap will make it easy to put this bolt in.
Put the fenders on. Fit it to the cowl, then go down the back of the fender, matching the style line in the door for height, and adjusting the fender in and out so the door front edge does not hit the fender. Then go down the top of the fender, checking that the hood gap stays OK. You may have to leave the bolt on top closest to the front loose. Start the bolt on the bottom front of the fender, the one by the parking light. Start the two bolts in the headlight bucket. With the door gap OK, and the hood gap OK, you can move the front bottom of the fender around to match the lower core suport rail, and finally tighten the bolts on the front of the fender.
Then you can install the front apron.
I have found it is handy just to go and buy a box of 1/4-28 bolts. Buy 1/2 inch, and 3/4 inch, and a few 1" ones.
The body on your 521 uses all SAE bolts.
Get the following taps. 8-32, 10-32. 1/4-28, 5/16-24. Those are the common sizes on the body of a 521.
The brake lines on a 521 are 3/16, with 3/8-24 flare nuts. Most other Datsun brake cylinders use a M10-1.0 flare nut.
Again, I am not saying you are doing it wrong. You have some really nice work on the VW's. you obviusly undertstand fitting body parts. Just trying to help other people who see this thread, and maybe give you a few Datsun specific items you nay not be aware of yet.
Go to this page, scroll down to the Datsun 521 section, and down load the factory service manuals. They really are pretty good.