[4x4Poet]

Originally Posted by CG

what will i be carrying? loads of pavers and bricks, loads of drywall, loads of lumber, loads of top soil etc etc... or pulling my 61 lincoln on a car trailer or any other 67-72 trucks i cant live without.

This screams 3/4 ton full floater rear axle, though not necessarily the one under there now. An Eaton, D60, or 14bFF would do fine.
The 12-bolts are strong, so if you only intend to *occasionally* haul pavers, bricks, top soil, drywall, loads of lumber, and other *lightweight* items, then maybe the 1/2 ton axles would be fine. Especially if you just put put around town with the heavy stuff. If you stick with the 12-bolt, replace the axle bearings as a preventative measure and use synthetic gear oil.
The front discs would be nice to have, as well. Tough call without knowing exactly now much heavy hauling will be done. Also, the 12-bolt has more gearing options and more locker or limited slip options than anything but the front D44s or a D60.

i also have a complete 71 4x4 1/2 ton chassis. with this i could transfer the 2.5 lift softride springs off my current ride, or just use the 3/4 ton springs off the 68 chassis?. also i would use my stock style rallies with the 33 x 12.50. i already have. would helper springs or helper air bags make this 1/2 ton as sturdy as the 3/4 ton?

I would avoid air bags on a work truck. The frame is not up to airbag pressure under a full load of bricks. I'm not fan of helper springs, especially when they ruin the ride, but they have their place. Swap springs as you please, but know that most lift springs aren't up to heavy loads. Your call. 33s would be fine but probably need a little suspension lift if you expect to seriously sag the bed under a heavy load. Light fender trimming is also an option and usually the best solution, but it can change the looks. Personal preference.
What? Looks can be important.

as far as drivetrain it will be using the 71 transfer case and automatic tranny. i have a very nice professionally rebuilt 350 small block i can put in or i have a 68 427 4 bolt i can put in (not a tall deck). gas mileage isnt a consideration....

I wish I had your engine choices. I'd love to run a 427. The NP205 is up to the task, for sure. The TH350 can be upgraded to handle heavy towing or loads, but an upgraded TH400 would be better [Captain obvious, here].
If you stick with the TH350, you save costs since you already have the proper adapter for the xfer case and also won't have to swap in a TH400 compatible input shaft into the NP205. No brainer, IMO. Set the strength issues aside until the truck proves that you need more tranny beef.

Sounds like a great project. I just picked up a '72 K20 with the Eaton rear axle. Great clunker, but it will serve my needs. My first 67-72 pickup, too.