looking to go with the widest possible wheel and tire for my 68 chevelle. seem to have hard time finding what will fit under wheel well.

Just did this on my '69. Old tire/wheel combo was 15x8 Weld ProStar with 4.5" backspacing, with a 275/60/15 Mickey Thompson drag radial. Fit fine, no issues.

Changed to 15x10 with 5.5" backspacing Weld ProStar with 5.5" backspacing, with
Mickey Thompson 295/65/15. Now have slight clearance issue on inner wheelhouse on passenger side (slight massaging will take care of this), and the lips on the wheel openings will need to be rolled.

15X8 4.5 backspacing 275/60-15
No problems
Dave

Check this out: http://www.chevelles.com/database/

Should give you some ideas of what others have fit. Personally, I like the 295/50x15's on a 15x10 wheel with 5.5BS. No fitment issues.

15x8 4.5 backspace 295/50/15
http://www.chevelles.com/showroom/data/500/100_0250_1_2_.JPG
http://www.chevelles.com/showroom/data/500/100_0253_1_2_.JPG

15x10" 5.5BS and 325/50R15's do fit my 69 with rollng fenders and hitting in inner fenders a bit. I installed a huge front sway bar and KYB shocks to stiffen up and ensure no body roll. It worked and they fit good and look sexy!;)

Bill, You have no issues?? Seems like they should hit. I would love to go to them if I can without cutting, rolling or anything.
Bill

Bill,I put quarters on the car about 5 years ago and I had a real wide lip(about 1 1/4").I trimed off about 3/4".I have about 1/2" of lip left.I have stock springs in the rear.
If you only have 1/2 to 3/4" of lip they should clear with no problem.

If you run 17" or bigger wheels, you can go 11" wide with 6-1/8" backspace and 315mm tire

If you run smaller wheels such as 15" you're limited to 10" wide with 5-1/2" backspace and 295mm tire due to interference with the lower shock mount.