Need help with 2 issues but same place!

If you want to put a 275 60 15 under the car, the only welding you need to do is weld on some new spring perches. All you need is the kit that moves the springs in 1/2" on each side, with the offset rear spring hangers. Mancini sells it. That kit, with the correct wheel back spacing will get you what you want without cutting and welding on the frame........not that there is anything wrong with that.