What about a resonator exhaust tip??? We put one on my Brother-in-laws Cummings after he straight piped it, the neighbors and my sister couldn't stand it at 4:30 in the morning when he was warming it up before he headed off to work.
I'm pretty sure he got it at the local Autozone. It dropped the tone down a few notches and really helped with the overall loudness. It was only $20-30. May be worth a try before you spend another $100+ on a muffler.
With a 40 series on there, he will need a quieter muffler. Swap to a 70 series, sell the 40, and deal with the cost. It will be the proper solution.
