Hey Rude72,
I am running a very similar combo.
1972 Duster
340 with 284/484 Purple cam
750 Edelbrock on LD340 intake
TTI shorty headers, dual 2.5” exhaust
3200 stall FTI Performance torque converter
727-A non lockup trans
8-3/4 Sure grip with 3.91 gears
255/60r15 (27” tall)
Edited to add more due to a glitch:
My car responds well to the foot with the 3.91s and requires no downshift if already going 40+ mph when mashing the gas as it is already in top gear and it’s powerband, putting me nicely back in my seat. Highway is 3500-4000 rpm depending on if uphill or downhill. And no shortage of passing power if needed because engine is good to 6000 rpm. I would not go with any less gear in this setup, I think 3.91 is the sweet spot.