I've got a new in box Thrush glasspack muffler, 2.5" inlet, #24214. These make great pre-mufflers/resonators. $18 plus shipping

Here's the specs:

Case Shape: Round
Inlet Diameter (in): 2.500 in.
Inlet Quantity: Single
Inlet Location: Center
Outlet Diameter (in): 2.500 in.
Outlet Quantity: Single
Outlet Location: Center
Internal Construction: Glasspack
Muffler Material: Steel
Muffler Finish: Red
Case Length (in): 23.000 in.
Case Outside Diameter (in): 4.000 in.
Overall Length (in): 28.000 in.