My 451 has 3" with an X-pipe back to the mufflers so far. MO requires tailpipes of some sort. I'm still waffling over whether to make 3" or 2.5" tailpipes. The smaller tailpipes will be quieter and easier to route over the axle and won't hurt flow much.

I think 2.5" from the headers back is too small for a 470 unless it's built for street "stump-puller" (low rpm torque).