First off, your 350 should have enough power to deliver plenty of RPM's to the tranny and your 3 speed should have a 1:1 ratio for 3rd gear, so this will not limit your speed. therefore the issue is probably the rearend.
Pull the rear Dif cover and count the teeth on both the Ring gear and the Pinion gear. Divide the number of teeth on the ring gear by the number of teeth on the pinion gear and you will have the gear ratio for the rear end in your truck.
I ran a 400 SBC w/TH-400 and 3.73 gears and had no trouble accumulating speeding tickets from the California highway patrol in my old 1963 C10. That ratio should have plenty of potential for you as well.
You just need to figure what you're starting with to know what you can physically swap in without having to change the carrier. Not all gears will just bolt up.
