If a guy wanted to keep it all Cadillac, he might look into transplanting a '68 to '76 Cadillac 472 or 500 cubic inch motor complete with TH400 auto trans or even adapt a later 700R4 behind the motor for the 4th overdrive gear. Anything and everything is available for these motors, including aluminum cylinder heads.
http://www.crankshaftcoalition.com/w...gine_Knowledge
http://www.crankshaftcoalition.com/w...e_500_Cadillac
http://www.500cid.com/
http://www.cad500parts.com/
http://www.cadillacperformanceparts.com/