I think '72 & later rears are 5-lug, while your truck is six-lug.
Tranny isn't going to change your top-speed (unless it's a 3 sp overdrive (optional) or the later T-5 5-speed from an S-10).
My '72 C-10 has 3.73 rear, 3-speed, and 250 six, and will cruise @ 60 all day, will do at least 80 flat-out.....
A ring & pinion chnage might be your answer.
Frank McMullen
