i have just installed a 509 Dart big block roller motor in my '64. Car has an m-20 that is rebuilt and has not had issues with LS-6 454 i previously had in it. I am told the 700 hp Dart will destroy the m-20 4 spd. Has anyone done this swap with a high horsepower motor? i understand the drive shaft must be shortened. Other wise tremec has a bold in kit. I am looking at the TKO 600 5 spd with .82 5th. I have 3:36 rear end. Newer c-5 and c-6 manuals require too much surgery. The tremec looks like a good bet.