Option for budget manual transmission upgrade

[D-n-Tyke]

So my 18 yr old son joined the addiction this weekend. Went and bought himself a 1972 C20 with a 350/SM465. We were told that 3rd in the trans is junk (works at slow speeds but we haven't had it up to speed yet though). So since he already wanted overdeive, he is looking into his option to upgrade to something with an overdrive. We are trying to keep it as budget friendly as possible, but also don't want to put something in that he is going to want to replace in 4 years.

As for the motor, starts up and runs. But she is leaving a nice smoke screen behind it. So lucky for him I have a 402 I built and ran for a couple years sitting in the garage. So whatever trans he decides to go to is getting the 402 in front of it.