Your truck is 4x4 correct?
If so then NO you will not need to do driveshaft mods. In the 1980's trucks with the 208 or 241 tcase the overall length of the tranny/tcase combo's are all the same. This way GM could use only 1 length of driveshafts for all the trucks. The length different length of components is equalled out in the length of the tranny to tcase adapter.
You will just need the Th350 tranny and tcase adapter from a truck with an NP208 from the early 80's. You may also need its dipstick, crossmember, and kickdown linkage.
Harley