I have been under the number one Barris Batmobile, in 2010, and I can verify the original chassis is still under it, and because the Ghia built body is welded to it, I doubt it could be changed. Engine was a 390.
This is me in the Barris fiberglass Batmobile number 4 (the 3'rd fiberglass Bat), at Fiberglass Freaks in Logansport, and I can also verify a Ford Chassis and a 427 dual 4 bbl side oiler.