Les, AMC sold High Performance (Groupe 19) parts over the counter well into the 70's. At least as long as they were campaigning cars at major race tracks, (which would have been the Matador at NASCAR, or off road racing with the Jeep.)
I kept my car mechanically stock for the 1st. 30 years. Finally I got tired of the car suffering from "detonation" thanks to the progressively lower and lower octane ratings the oil companies offered us. So I installed the better breathing, (but lower compression ratio 1972 Heads.) To make up for the compression ratio deficit, I installed an R4b, mild cam, and later designed free flowing exhaust manifolds. (All with AMC part #'s). Unfortunately any performance gain I should have realized, was lost thanks to the lower compression ratio. Still my 1/4 mile times remained exactly the same, but I could run pump gas without mixing in racing fuel. I did gain about 10 mph on the top end.