||Percy, I have owned a 1978 DiplomatIII since 1988 and have had to replace both exhaust manifolds over the years. Last year I broke down and replaced the right one. I got it a a place called Stevens. They specialize in cast manifolds. I have the number at work. They are a good resource for these. As I understand they were only used on RVs and 440 powered trucks. I also replaced the long exsisting exhaust pipes with larger diameter stainless pipe and two Flowmaster turbo mufflers exiting out both sides. Now at stop signs it sounds like a built Cuda and the performance noticably picked up over the stock system. Maybe your cracked manifold is creating the heat you mentioned on your foot. Crossed the united states three times in mine and only got saddle sores.
Happy trails, Rex Urban