Muffler options ?....

IMO, the best he can do cheaply and effectively at a reasonable price is a 1-3/4 header primary pipe size into a 2-1/2 exhaust with a pair of mufflers each flowing north of 460 cfm. This should be the basic minimum for a basic set up. The addition of an H or X pipe should be introduced.

He said he basically copies the MP 380 crate engine. They routinely dynoed north of 400hp. Assume more power for head room. Calculate at 420hp.
420 X 2.2 for total cfm needed for a zero loss. Divide by 2 for each mufflers minimum flow need for as close to a zero loss system.

While exhaust science can get to a long and drawn out thread, the math and pointing fingers at various methods and designs and muffler configurations, his bottom line requested was a quite muffler.