The car is a conversion from automatic, yes? It sounds like the transmission, flywheel, pp and throwout bearings all must be matched together and it seems if one is wrong in the mix you might be in trouble. It sounds to me as if you have one element wrong. Could be pp or t/o bearing....

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.