exhaust exhaust exhaust uggggggghhhh
I think I know the sound you're talking about, my buddy has a '73 B250 van with a stock 360 and duals off the stock manifolds, no crossover, I think 2-2.25" and has rattly Dynomax Super Turbos. It's got that light "grunty" truck sound where you can tell the engine is stock as hell.
I think you should add a crossover first of all (X-pipe if you can), that will really smooth out the sound and make it more sports-car-like. And what size pipe is the rest of the exhaust? 2" all the way would sound like crap no matter what mufflers you had.