Don't know any particulars regarding the c-series body to d-series frame swap, other than to point out the obvious. Going from pickup to pickup would be easier. There are likely differences in the frames between pickups and t/a's. Probably not insurmountable. Is the t/a a 3/4 or 1/2 ton version? You might find that swapping over the drive train minus auto trans and running gear from the t/a would be more feasible in the long run.
As for the second question, your wide ratio 4-speed will mate to the 392. You should have the flywheel from the 304 neutral-balanced at a machine shop to prevent vibration before mounting it behind the 392. There could be a difference in overall length between the two transmissions. If so, that will have to be accounted for when coupling to the divorced t-case. Both prop shafts should be unaffected by this swap.