Slants weren't built to rumble. Don't waste time tryin to make one sounds like a V8. If you want a V8, get one. To me, a good hot slant sounds like a heard of pissed off killer bees. I kinda like that sound.
Best slant I ever heard was in a rail at the Antique Drags at California Speedway, it sounded exactly like that herd of bees until the light turned green. Then it sounded like the world's largest man bending over and splitting the *** out of the world's largest pair of Levi's. Impressive to say the least.
My wagon has a 14" glasspack on it and sounds pretty darn good. It does drone a bit at freeway speeds, but its not overbearing, and the wagon's got no carpets in it to dampen the sound.