Well gee...................
You've posted a question nobody can answer. We don't know WHAT you want for RPM, and we don't know what RPM you run now "when cruising" nor do we know "what cruising" MEANS
Here's the deal:
ALL traditional 4 speeds, 3 speeds, and Torqueflites are 1:1 in high gear, which means that "cruising" RPM is determined by
rear gear ratio and tire size.
YOU have to determine what "cruising" means to YOU. 45? 65? 70? 140?
There are plenty of online calculators
mph, rpm, calculator - Google Search
So take your tire size, the rear ratio you have now, and decide what mph you want to call "cruise" and calculate it, then decide how much HIGHER RPM you can "handle" and plug that into the formula. That will tell you what gear you think you want.