you want suggestions...
DONT put a caprice frame in, almost all car frames mean you have to hack out your steps because the center cab section is just too narrow to match up to any car frame.
Also, as for the motor, unless your going ultra-modern like an LS engine, i would keep the vintage poncho just the way you have it.. very unique - EVERYONE has a 350... (for this reason i put a pontiac in my 55' chevy
) the original air bath and valve covers are awesome, and can probably be cheaply touched up.
lastly, if you plan on driving alot, just change the front suspension first and go from there. Mustang II kits as well as camaro swaps are common and not too difficult, and WAY less guesswork than trying to make a car frame work.
just suggestions...