With a mini tub and spring relocation you can do 13.5 inch wide ones.
With just the 3/4 inch spring relocation and stock tubs I'd say 275 width if you want them tucked still. Might have to do a little trimming of the fender lip. I had 255's with no offset springs on my 69 and I had to trim the fender lip a little and had about 1/4 of clearance with the fender lip and leaf springs.
