I may be a newbie here but I agree with Don's statements about the 68 model year being a bastard child for most muscle cars. I don't know why but they seem to be.
I found a couple 69 GTO drivers for you to look at that compare favorably if not better than the white '68 GTO you have been looking at.
For $24,500 you can buy a real PHS documented '69 Judge
http://cgi.ebay.com/ebaymotors/1969-PONT...=item53e2436b4e
For $22,000 you can get a super nice '69 GTO that was owned by the same guy for 30 years.
http://minneapolis.craigslist.org/an...828019475.html
Neither of these cars are mine however I feel they both appear to be excellent candidates for you. Green isn't so bad when the paint is new.