Yes, if you are using the "newer" sizing, 255 60 15 is probably it for a 69. I had a KILLER pair of 275 60 15 Hoosiers, and they were so close to rubbing with the car sitting still, I knew they would rub as soon as I started driving it. Broke my heart to sell them.
I have 255 60 15 on the back right now, and there are no rubbing issues. Your stock rally wheel should work fine with that size. 255 means it is just over 10 inches wide at teh section width, not tread width. So a 9 inch wide slick has about a 10 inch section width. As for heighth, the 255 60 15 is 27 inches tall. If you can find a slick in the size stated above, 28 x 9, you should still be ok. 27 x 9 would also work.