What is the wideest possible tire in a 15"
that I can fit under my 1969 chevelle
[I have cut the wheel well outer edge about 1/4" and I am willing to custom make any
thickness of spacers (left and right)
any help would be great.

396Ratt, run a search on wide tires and you'll find your answer. I know for my '67 it is 275/60/15 with a 4 3/4" offset.

I am running 15 x 10 SS wheels from Wheel Vintiques with 295/50/15 BFG tires on the rear of my 69, but they are TIGHT (about 1/2 inch of clearance to the fender). I think the wheels had 5 inch offset, and an additional 1/2 inch would work better, they have plenty of clearance inboard.