Need help with gear choice

Car info
73 Scamp
360 Magnum stock internals
Hedman Headers, Air Gap intake and Eddy 600 carb
2800 stall converter in 727
26" tires

I have a 8 1/4 with 2.45 gears now

I just pulled a 8 1/4 (27 spline) out of a dakota that is suregrip with 3.90 gear. I also have a set of 3.21 gears that I could use. The gear I have now is a dog out of the hole and I have converter slippage. I mainly cruise around town but occasionally on the interstate for a few miles. The dilemma is I don't want to overheat the tranny but I don't want the rpm's thru the roof either. What would you guys do?

BTW I know the carriers are different and I don't have 3.55s so it will be 3.21 or 3.90