let me get this straight, your son is building a street rod and wants to use your 4x4 axle? remember,it's 6 lug. if you want a 5 lug 8" you need to find an earlier 2wd('69-78) as most if not all of them had the 5 lug 8", most 2wds got the 7.5 starting in the early 80s. however, just about every 2wd 4 speed or 1 ton rated truck came with the 8" too.

to answer your question about gears, i know the 8" came in 3.08 in some 2wd 4 speeds and also in 3.90. i DO know that 7.5s in 2wds were available with 3.42, 3.54 and 3.73 ratios, but i'm not entirely sure if you can get the 8" in a 3.54 ratio in the US(i've heard australia has them though)

all 8" diffs are interchangeable from '69-current. the only vehicles i know of that still have the 8" are 4runners, the last of them in trucks were used in '95