65 66 and 67 will interchange.
1968-72
Any 1978-87 El Camino or GMC Caballero
64-65-66-67 chevelle.
A standard Chevelle frame will not fit an El Camino as it was based on the station wagon frame
No
yes it will, they are the same frames.
Yes, 1968-1972 had the same frames.
Yes
If the owner's manual doesn't tell you, you need to contact the maker.
some have coil spings some have leaf spring but will interchange
In those years they were not interchangeable.
Yes, '68-'72 El Caminos (Gen 3) are the same body from the firewall back. Frames are the same.