Datsbad has had 2 '69s with 20s on the back. Both used a 255/35-20 tire. One had grey centered TTIIs that were 20x8 with 5" of backspace the other had 20x8.5s with 5.25" backspace. Both cars were lowered at least 3" and had Baer rear discs with no rubbing issues. There is a member here Sandros with a '69 that is running 20x9.5 Ralley Wheels with I believe at least a 265 or maybe even a 285/30 rear tire. I don't know if they rub. I think on a very low car at the least they would be very tight and maybe some minor rubbing over curb cuts. You may want to PM him. Good luck.