IMO, keep the 67-72 frame. everything is bolt on vs fabricating body mounts. Trailing arms will ride, lower, and bag way easier than a leaf spring setup. front suspension is cheaper to lower/modify with stock setup.
You can get junk yard parts for cheap.
73-87 front suspension is bolt on, 5 lug disk brake, all you need to do is elongate the two rear mounting holes on the frame and the entire front end lines right up. 5 lug axle shafts are $300, Drop spindles are $150-200, drop coils...fuck it just cut yours. Rear blocks and drop springs are cheap.
It really depends on what you're going for, if you want to do LS swap, and do disk brakes all at once, than sure, it's doable, but will take a lot of fab work to make body mounts that will work on the 67-72. Then all of the fab work to adapt the 67-72 core support to hold all of the NBS stuff, plus adapt the firewall to work with NBS brakes, HVAC, etc. It's not an easy task.