I was looking around for someone with some insight on running/changing my 521 hubs from the 6x5.5 to 4x114.3..
Are the spindles the same to the 510's or another datsun?
The cars stopped using those types of "spindles" in the late 60s. You need to be thinking Roadster or 410/411 (something with a full frame), not 510 (unibody that uses struts). Those would be the best bet for swapping the front to 4x114.3. Swapping on a late 2000 Roadster front suspension would be ideal because of the disc brakes.
The rear is just a case of swapping in a car rear axle assembly. An '80-83 200SX has nice gears, factory disc brakes, and should be a decent width for the truck.