rim width?
 

Is 15x8 the widest rim i can get under my M-body or with the right back spacing is it possible to use 15x10 ?

depending on how much room you have between the tire and the fener well on both sides as well as the suspension components. it is possible to do it but you may have to roll the fender to clear them and possibly mini tub the inner wheel well.

CAN you fit it without modifying anything I would probably guess yes but how much smaller of a tire are you going to have to run to fit it would be my concern. I have 18x9.5" on a 95 nissan but had to go with a stretched 225. where as my newport I went with an 18x9 and have a 275 with room to spare. Just depends what look you want and then measure measure measure. local tire shops should be able to help you with the measurement aspect if you need. I took advantage of that for the newport. Thats my NON PROFESSIONAL suggestion. Good luck!