Exhaust manifold leak and still misfiring

Still chasing this multiple misfire deal. We had inconclusive results with the leakdown test which will be performed again this weekend under more controlled conditions. The CAT was removed only to find that it looked clean and un plugged (you could see light through the mesh inside) and very little soot came out when hitting it with 120psi of compressed air. While runing some seafoam through it to decarbon the engine I found a sizeable exhaust manifold leak right where they all crack at the weld. Anyone ever heard of misfiring problems related to an exhaust manifold leak or any other problems associated with an exhaust manifold leak? We came up with a "replacement pipe" for the CAT (for offroad use only of course) and it still manages to heat up the floor something fierce. I could have sworn it was a plugged up CAT that was causing the heat issue but I think we may be missing some heat shielding and the carpet insulation masked the rest of the heat ? ? ? ? ?