There is no way you can run a 30 or 40 series 20" tire in a 67-or 68 with out fender surgery. Prodigy (Frank) is running a 25 series and I think it is going to be a squeeze. I am planning on 345/30/19's and it will be a squeeze at 27.5 inch tire. 13" wide rims with a rolled lip gives me 3/4" each side which I don't think is going to be enough unless I go solid bushing rear so there is no lateral movement.
There is a pic of a Red 67 that was at SEMA in the Pypes booth and has been posted here with 22's out back. Wheels are sucked way in and has real tubs. CLick on link for pic
Mike