Without looking up "Plymouth Valiant" on Wikipedia or looking at Mymopar, etc, as I recall the 60-62 A-body cars had unique structural stampings; obviously the bodies were unique.
Mom had a '60 Valiant that I drove daily, and a girlfriend had a 64 Valiant, and though they drove & handled similarly, I don't think much was the same.
The Slant 6, maybe some mechanical & suspension items, and maybe some electrical stuff likely transferred.
The 1st-gens dominated that NASCAR series, and IIRC Mopar advertised the '63 (2nd-gen) as being all-new and tested for a million miles.
I've enjoyed driving every generation of A-body.