The Older Caddy 472s and 500s up to 1972 are the better ones to build . I did work on a 1972 with a 472 . I think the thing with the caddy engines is not so much it's HP . But it's that engines Torque . that what get's the job done . The older ones have the better heads . But even the newer 500 Caddy with a mild camshaft and a better intake with a curved HEI . And a nice Dual Exhaust sys Will be a nice Low RPM street engine . If you want more info on Caddy engines I can post it . As for that TH400 . It's a Long Tail type . You can use them and they do have HD cases . But you will need a short drive shaft .