I did find this answer to the Heavy Chevy question, posted by a Dale M on Yahoo Answers.
"The Heavy Chevy was an option (RPO YF3) on 71 and 72 Chevelles. Only available on the 13437 or 1C37 base Chevelle models, never on the Malibu. Since it was not a separate series, there's nothing on the trim tag or VIN indicating the option - no more than there is for any other option like a radio or air conditioning.
It is known that for 71, the option was ordered 6,727 times and for 72, 9,508 times.
Both years could be ordered with either optional 350, 2-bbl or 4-bbl - both 2-bolt main.
Standard rear end was dependant on transmission type and engine.
350-2 w/automatic - 2.56, w/4-speed 3.36
350-4 w/automatic - 2.73, w/4-speed 3.31
Positraction was *never* standard fare on any Chevelle, it was always an option.
There is no such thing as absolute value in this world. You can only estimate what a thing is worth to you. Since original engine and transmission are gone, unless you have paperwork showing the HC option, any value will depend on the quality of work done to bring the car back to life - and, sadly, probably little to no return on investment if that's what you're looking at.
Build the car and have fun!
Source(s):
http://www.chevellestuff.com for 64-72 Chevelle information."