I would look at cam and gear changes to get the power you want. The truck is shaped like a brick its not going to do major top speed runs.
What is the goal top speed racing like they do inthe desert. Or 1/4 mile cause it aint going near that in the quarter in street form. But you cando some crazy mods to the 4.3 v6 it will make a crap load of power with a bunch of different combos. Nascar had to limit theuseof v6 becasue they were going so fast made almost as much power as the v8 but with less weight.
Most cars are not geared for top speed. In point my honda will easily top 4th gear at about 130 phm but 5th is so far away the motor is out of the powerband and will not hold top speed unless i hit the boost curve just right. Nut for the most part it does not ever hit the speed the engine and car can do.
Most top speed cars have tall gears and hit top end in 3 or 4th to keep the gear up. But some use over drive and short gears jjst depends on the combo and rpm you can hit at what speed and gear.