Depends on a few things. I am running 245 front and 255 rear 17" rims. The rims were made to fit the largest tires I could under the car. I had to roll the fenders and they slightly scrape in the rears (Bad rolling job). The problem with fitting big tires in the rear is the leaf spring is very close to the tire and unless you use a narrow spring or coil over kit you are limited. I have seen a car running 275's on the rear but it was very tight. The front is another issue as you move the rim inwards you have suspension issues with ball joint clearance and also turing radius depending on suspension type and rim diameters. 17" rims look good on the car and do allow a good selection of tires and brakes, 15,16, not so much, and don't know if bigger look right on the car. So all in all you can stuff a good sized tire but you will have to get rims that are offset right and do a little fender rolling.

did you ever find headers? what did you do for motor mounts and an oil pan?