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I have a '69 BB, 4 spd that I put the 2 1/2" Mid America chambered exhausts on. As I recall, their catalog did not have a number listed for the chambered listed for my application, but the tech person led me to the proper one. Something like their generic chambered.
The only modification I made was at the muffler shop where we welded on a full 2 1/2" tip. This was instead of the necked down pipe at the end to accomodate the stock exhaust tip.