it should be any 4 bolt main small block, like a 305 350 383 400 327 you could even upgrade to a big block, but you may run into some trouble with engine mounts.
The 400 wasn't available in the El Camino in '78.
No.
yes
Yes
Yes
No, they will not interchange.
yup...
A standard Chevelle frame will not fit an El Camino as it was based on the station wagon frame
Doors from a 1978-87 El Camino or GMC Caballero
no because it poops butterflys and rainbows !:D
Yes. They should bolt right in.
You don't...