3.90 YES or NO?

I have a 318 with a MP 284/512 solid cam. It has a Performer RPM intake, Holley 650 double pumper, 9.8 compression and headers. The tires are 255/60r15. It's a street car, and I almost don't drive it on highways.

As you may think I don't have lots of low end torque, and the engine starts to run nice at about 2500 rpm. My rear end is a 3.15, and with the big tires it's not the best choice. A friend of mine has a 3.90 in his stock 318 and he proposed a rear end swap.

Do you guys think it's a good idea?

Thanks