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

[donut]

first thing I always check, gas cap/s. Loosen them, if it runs better then it's a vent issue.
Is it rpm or speed based? Can you hold it in 2nd gear and replicate the problem?

I only ask because my timing chain is stretched and that throws timing all wonky. 118k, wouldn't surprise me.
Have you pumped fuel from your fuel line to a can to see how much and if it's pumping well?