New rear end/differential??

[Coltronist]

Hello my friends,
I have a 63 c10. I just bought a 350 motor that I'm turning into a 383, and a th350 Trans.

I want to try and get this truck to be able to cruise at 80mph and with my little knowledge I thought just putting a new Trans in would work, so I thought of buying a 700r4.

My neighbor from down the street has an apache and was telling me that it's not so much the Trans I need to replace, but the rear end.

Here is the question:

If I replace the rear end, will it make that difference with the th350? Will I be able to cruise at 80 by doing so? And if so, what rear end should I look for that I can use on my truck?

I'm going to school for engines and transmissions starting tomorrow so I don't know too much, just seeking your wisdom

Thanks for the help!
-Colton