exaust size

I have 3" on my 72 w/383 all the way over the rear axle and coming out after the rear of the tire and start of the lower quarter panel.

Not sure what the rule is but I would think once you start getting around 550 hp you would need the 3". Mine might go 520-525 hp and I wanted 3" and had it on my last 71 w/406. The 3" will get louder with the larger diameter pipe.

If you don't want loud then stay away from anything near 3".

I think they say 2 1/2 inch up to around 500 horse . 3 inch from there but i dont know what the cut off range for the 3 inch is , hope this helps a little, Good Luck.