Personally, I'd run some sort of mufflers ahead of the rear axle, then go up & over, then a pair of straight-throughs (6" dia. x 18" long) between the frame rail & the gas tank on an E-Body. I like the expansion chamber in a couple of places in a full exhaust like that. About the thinnest oval mufflers you will find with 2.5" internals is going to be 4-1/4" thick - pretty standard.