does anyone know the biggest tire you can run on a 8" rim on the rear of a 65,with a stock width rear-end,I do not want the quarter panel lip to be modified .

Without any modification I know that a 275/60/15 on an 8" with 5" backspace will not fit. You will have to go to the next size down.

I installed some 255/60/15s with 5" back space on the rear of my 64 and could not get my finger between the tire and the wheel well lip. There was plenty of room on the inside. I think a 5 1/2" back space would work. I installed the same tires & wheels on the back of the 72, I could get my hand between the tire and wheel well lip.
BTW the 64 has a 66 short rear end, Same as the stock 64/65.
I know 235/60/15s will fit the 64.
Bob J.

i just put my 255/45/17 with a 8" wheel and 4 3/4 BS on my 65 with room to spare,and this size will work in front also

I had 245/50R16's on my 65. Rolled the well lips and pulled the inner fenders-lowered real low.

You need to first look at which axle is being used when any recommendation is given. A 68 and later axle is 1-1/4" wider. Many people put a 12 bolt from a later model in and forget that this affects the backspacing. I run a 255 tire on a 8X17 with 5" backspace wheel with a stock 65 axle. I have 1" clearance on teh inside and 3/4" clerance on the outside without modifying the wheel lip.

You can easily measure your own spacing requirements on your car. If your interested in measuring you car, do a search on my userid. I have posted the method many times.

I have 245-50-16's on my 65 using 16 x 8 TTII's with 5" of backspacing. I got about a fingers width of room on the outside. I believe that 5-1/4" of backspacing would have been best, 255's should fit easily with that backspacing. Max backspacing on 8" TTII's is 5-1/4"

Heres a pic.

http://www.chevelles.com/showroom/Jeff65SS/65-side1.jpg

Jeff