Mufflers are for street cars
If your intention is truly just to quiet the interior then add a turn down to your collectors and point them where the air would hit the ground out side the body line. This alone, (if you have not already done this) will make a huge difference due to the vibration not being transmitted to the sheet metal interior and driving you crazy. This muffler idea my be over kill and slow you down in the process. Just wanted to offer an effective and inexpensive option that may solve your complaint without the weight, bulk and cost of mufflers.