What size now?

Did you mini tub it ? You cant go very wide on a 7" rim and if the wheel wells were stretch you did gain that much. Maybe another inch tops.

I know guys that run a 325-50-15 or 28X13.5 on a 15X10 rim with mini tubbed cars. Other wise in the stock wheel well you should be able to stuff a 275-60-15 in it at the most. I measured a 255-60-15 fitting perfect in the stock wheel wells on a 15X8 rim with 5" back spacing after moving the springs.