What size rim and back-spacing??

[Sikmalibu]

I was wondering if anyone knows if I can fit a 15x10 rim if I do a frame notch to the seam. If so what is the backspacing. I plan on doing this very soon but if I can't squeeze a bigger rim than whats the point. I have a 15x8 with 3 1/2 backspacing now with a 29x10x15 slick that just misses the frame. Any input would greatly be appreciated.