what would limit my top speed, truck wont sustain 64mph?

[324matt]

I finally got my truck all put back together, and, besides installing 2 new fuel tanks and sending units and the fuel gauge not functioning properly, I cannot seem to get the truck up to speed. It runs good through 1st 2nd and 3rd gears. It starts right up and idles great. it pulls good for a stock 350 with 118k, right up until I get going about 62mph. Then it sputters. Then at 63 or 64mph it starts dying out like it is starving for fuel. If I let off the gas pedal it comes back alive and will start to slow down. I waited until I coasted down to 50mph and then floored the pedal and it raced right back up to 63 or 64 and started to die then loose speed and come back on. gain speed, die, come back on. Can anyone give me clue on what to adjust? or check? is there a rev limiter or a governor? its a 85 k20 with qjet 350 and th400. 4.10 gears. all stock. stock tires.

Im stumped.