383 to 727 help - 66 Charger

So a couple things for you and your friend to verify;

What was the original auto trans? 904 or 727 and was the flex plate used from the original small block crank? The flex plate from a small block will most likely (there are a few exceptions)bolt up to the big block crank regardless of the auto trans however the 904 flex plate is smaller and will not bolt up to the 727 stock converter. If the original auto trans was a 727 then the flex plate should bolt up to the 727 converter but as others have noted the bolt holes will only line up when clocked to the correct possition (you have a 1 in 4 chance of getting it right the first time!)

Verify the 727 to ensure the bell bolt pattern is correct as they are different for small block and big block (no stock 904's with big block pattern)

Verify that the 727 convertor is neutral balanced - no extra large weights welded to it (there may be small ones to balance)

The above not verified necisarily in that order lol

There may be other differences but I think these are the major ones for the typical years and assuming we know the engines in question are in fact 318 and 383 internaly balanced (I cannot recall if the 1971 383 cast crank engine is externally balanced or not)