What rpm does 3:91 gears give at the end of the 1/4 mile

It depends on how much power you have. For example if you have 380 hp, a 3550 pound car with you in it, that gear and tire will get you about 5500 rpm. If you have 450 hp then you would be at 6100-6200. Unless you have well over 400 hp you don’t have near enough gear or too tall a tire. That will make the converter very inefficient. A 1 inch change in tire diameter can make a converter much more efficient.
With me in the car about you hit the weight right on the money and about 350 to 380 I’m guessing on the conservative side of HP. Are you saying I may need about a 27” tire.