You can buy new Ansen Sprint wheels from American Racing.
They offer a 15X8 with 4-1/2" backspace, would be a tight fit in the back with a stock-width '68 rear end since they'd sit out a bit further (about 1/2") than what would be ideal.
Too bad they only offer a 15X7 with 4" backspace as the only other 15" size.
You'd have to find some 'real' Ansens on the used market if you really want drag-race skinnies for the front (I would!).
http://www.americanracing.com/browse_vintage.asp
