He replaced the hearse 's tall top with the roof from a ' 57 Chevrolet station wagon , grafted on rear fenders and tailfins from a late- ' 40s / early ' 50s Cadillac , split the one-piece rear hatch to create a wagon-style clam-shell door and installed it all over the chassis and powertrain from a ' 67 Cadillac Fleetwood Brougham sedan .