1966 engine with 1981 904A

I have read 1967 and older slants cannot be used with 1968 and newer 904's without some modifications to end of crankshaft, or the plate..does anyone know if it just doesnt fit or whether it just works incorrectly?

Im asking because i have a 1966 engine( modified by other with very little info(casting number-2463430-17) and a 1981 904A (PK4202662X) ..i have put 200 miles or so on it, and previous owner said he had 600 miles on it.