Diff Ratio Dilema

Tire size plays into it as well as it changes the overall drive ratio. A truck with tall tires can get away with a higher gear ratio compared to a car as they work out to be fairly equivalent when you combine the tire size and gear ratio. I'm running a 3.55 setup with my fairly small factory-ish size tires. I put in a T56 with a 0.5:1 overdrive though to keep the revs down. I imagine your 5 speed is probably ~0.7:1 overdrive? That would be like my 5th gear and I cruise around 2000 rpm at 70 if that helps.

If you plan to do a lot of highway miles anything on the lower side of 3-3.5:1 would probably be a pretty good fit. If you want more fun and punch then 3.5-4:1 would likely be the way to go.