Big Block muffler choice

If you’ve already got the 3” pipe, then I would reduce to 2.5 after the mufflers. Or even after the resonators if you plan to run them.

The 3” pipe also explains the loudness, it’s quite a dramatic difference between the two. I also don’t think there will be much of a power loss when reducing to 2.5 after the mufflers if you consider the velocity of the exhaust charge as it cools in the pipes.