Welcome to the site! Before I carved up that RR, I'd post it on FBBO and see what you're offered. You might get enough to replace the driveline you'd be selling and even make a couple of bucks. As for the 3:91's; I ran a set in my Valiant with a 340 4spd and it'd turn somewhere around 36-3700 rpm at 60-70 mph. In short, it wails and the gas mileage is horrendous. It'd pass pretty near anything- except a gas station. Between the wear and tear on the engine and the gas mileage, I never did it again. But, that's just my
and it's your car-s, your choice. Have fun.