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If you want "originality" stick with the caddy 472. If you want drivability and parts availability go LS all your conversion stuff will have to be custom, but once made your good to go.
I know this is an old post, but I say Cadillac all the way and convert it to a TBI system off a 454 or Marine 454/502 engine if you want extra driveability. Q-Jet fed 472/500 CID Cadillac is a tough engine to beat for low-midrange torque and they actually do quite well on gas.