header back side exit exhaust

[gmc1971harwich]

ive got a 71 pickup with a 327. is there anyway i can make my exhaust system more quiet without having to replace the whole thing the whole exhaust is still in good shape. its got headers into 2.5" header pipes into 2.5 inlet/outlet thrush super turbos then 2.5" side exit pipes . i drive it every day and its just soo darn loud inside the cab and outside. can i add 2 more mufflers or do they still make resonators to go into exhaust systems? i dont have the cash for a whole new exhaust. i was quoted a price of $225 just for a couple of tailpipes, and i dont have it to spend on it. i would rather keep the exhaust and add something in.