I went down to metal on my 80 malibu project - just like you, I was misinformed and had the wrong idea. I encountered factory bondo / and sealant which made my life a whole lot worse.
having done my own body work myself, I would never do it again unless I had all the proper tools that pro's use, ie. screw compressor so I could use and air board for quality sanding.
G-bodies are so cheap and easy - you may want to consider doing a frame off resto, sending the shell out to get walnut blasted, get the frame blasted and coated as well.
I'm only going off of what I know and typical capabilities of others, body work isn't the strength of 99% of the people, I had slight aptitude for it and still didn't turn out well. I would rather spend my time money and efforts elsewhere and hire a painter to do that work in the mean time.
you may want to check out a couple other forums:
www.bcgbody.com is just a community full of guys who give each other a hard time
www.maliburacing.com is a great technical website