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.
Dangit ratamous, you got another one right. What are we gonna do with you?