I believe I remembe reading here that on the 68-69's, the biggest that will fit under there is a 15x8 rim with 255/60R15 tires. I'll see if I can find the post. Phil
I don't think you can put 255's in the front without experiencing rubbing, if I recall what's been posted. In the back, yes. I'm putting on 245's all around this spring.
On my '69 GS 400 Conv. I'm using: New Buick Ralleys/goodies from Wheel Vintiques -rear 15x8 5"BS 275 60 R15 BFG -front 15x7 5"BS 235 60 R15 BFG Its tight and they seem fine. Looks great too. About an inch taller in the rear, great stance. Still have the 14" factory ralleys tucked away. Will snap some pics if anyones interested.