I am actually in the process now of swaping a 1950 chevy 3/4 ton truck onto a 123"WB (X cab 4wd)s10. So far everything is fitting like a dream. At the moment my truck has 3 inch lowering blocks and a pulled leaf in the rear for about 4 inches of drop and in the front i have 3 inches of drop from new torsion keys and am still figuring out how to get another few inches for that raked look.
Anyway, For the halfton AD truck i would go with the reg cab long box 2wd s10 frame. The WB on that is 118" and should be very very close to that of the original frame. The width of the 2wd is just a hair narrow. To compensate for this you can use a 4wd s10 rear axle (direct swap) which is about 5 inches wider or (what i am doing) a ford 8.8 rear from an explorer (basically bolts right up). In the front you can get some billet aluminum wheel spacers.

The hardest part about this swap is when you get to the radiator and radiator placement. What i did and what may others have done is to use the existing radiator core support from the AD truck and tilt it back slightly because the steering box is in the way.

All in all this swap is not very hard.

All the best,
Tyler