Absolutely one of the first things one should do after buying any 50's, 60's and early 70's Mopar is check the heat riser. They usually are not maintained, are often frozen, sometimes completely closed. Talk about a restricted exhaust. The one in my 56 Plymouth was frozen shut; removing it picked up eight tenths in the quarter. (OK, from 18.6 to 17.8 - but hey, if it was worth that much on a car whose stock 277 poly probably doesn't deliver much more than 100 hp to the wheels, imagine how badly a more powerful motor could be hurt.)
