geared way too low

[xratt]

im new to the forum, so bear with me. I just bought a 1966 c10, with a 350 sb, and trans. out of a 1982 chevy truck installed in it. They did the swap and sold off the project. So I have a lot to figure out. I will be back with lots of questions. I'm sure all these have been asked before, I just did not find the threads to answer this stuff yet, you can refer me to them if its easier.

First off, it has the 350 sb, and i think its a th400, i still have to verify that. The stock rear end is still in the truck. The original motor was the straight 6 and a 3 speed trans. With the new motor and trans, it shifts super early.
I will hit 3 gear by 20 MPH. So the motor revs out trying to go 55 and 60. Is this a rear end deal, geared super low? Or is the new trans not hooked up properly? I already found what i believe to be the kick down cable cut and hanging under the truck. If it is the rear end gearing, is it easier to regear it with different gears? Or just replace the whole rear end? Im assuming the 1982 rear end would work best. I need as narrow rear end as possible,cause he truck sits real low and the tires almost hit the bed now.

So any help you guys can give would be wonderful.

I will have plenty more questions as this truck progesses, so dont get tired of me asking dumb questions. I plan on keeping this truck a long time, as soon as i get it running decent it will be my daily driver