Okay, I know that its shaped like a brick, its not meant to go fast, but how fast have you guys been able get going in your square?
Far too many factors and options to even begin to answer. Yes the question presumably shows you're about 16 years old givertake a year or 2.
My answer is if you want to go fast, first concentrate on making the truck quick, and safe.
But a direct answer is, I recall road trips as a kid, doing 80mph for hours on end in a '74 C20 or '85 C10. Same with my old 1980 plow Blazer in 1990 ish.
Current truck on 35s and 4.10s is stable at 70mph and even 80 mph, but she's already running 3k rpms at 70, so going fast is an effort in financial futility, whereas a C10 with the right gears and in good shape would hold 100 all day (except for the pit stops every 90 min for gas!).