Cruisin with low gears ?

Your worries are valid.... but tell us more!
how loose is the converter? Or is it a lock up? Is it a stock driveshaft or something else?

I have a 200r4 lock up 3200ish stall, 4:10’s. If I don’t lock the converter and cruise in 4th too far below the stall rating for a bit I build quite a bit of heat. I actually plan to relocate my plate cooler off the radiator this winter... if I am locked up, or cruise at a more efficient rpm for the converter it’s a non issue. In my head the lock up is well worth it, it’s literlaly like 5th gear.

As far as driveshaft speed your RPM of the shaft should be whatever the engine RPM would be in 3rd with a given gear-set. I wouldn’t think 4:56 at 70 would be too crazy... If it’s got fresh U-joints and been balanced that will help for sure. However, I’d calla driveline shop to get their opinion as I know shaft material, length and diameter can effect their safe RPM. I wouldn’t mind buzzing past their suggested RPM from time to time but a long cruise might spell trouble.

Joe