63,
Doing the RPM calculations should only require tire size, the known gear ratio or a couple of other variables will tell you what your final product RPM would be. There is a link to a website on here somewhere. That site will help you figure out the calcs. I will post it if I find it.
Since you have an open drive line you could drop in a different rear end as well to get your cruising speed up. I don't know about 60's vintage rear ends, but wonder if a 78-90 caprice would be the right width? With a 4 speed, your rpms should drop.
Also, a tranny for your truck should be easy to find. I think $50 to $100 and you should have a good manual. I bought a whole S-10 with a T-5 for $100. The tranny now resides on the garage floor and the S-10 went to truck heaven.
What is wrong with your 3 speed by the way? If it is like a 66 my bro used to own, the shifter linkage is the problem and not the tranny? It might be an easy fix and then you could swap rear ends to get the RPMs down...just a thougth.
