1- The benefit of having wider wheels gives you the possibility tu run larger tires, improving the handling characteristics of your car, as well as it's appearance.
2- As for selecting 10" and 12" wide rims, it all depends on several factors as the rear spacing you choose, if the frame on your car is notched or not, how much you're willing to have the tires stick out of the body, etc...
I'd suggest getting T-Type rims redone instead since they're much more light weight. As far as what width you go with depends on how fast you want to go and again if your frame is notched and/or your fenders are rolled/tubbed.
A 10" wheel will NOT fir your car "normal", some mods will need to be done.
You can however, use an 8" wheel with the correct backspacing. The local street/strip cars can run into the 10's with 8" wheels and the appropiate tires.:smile:
If you are going for the "look", that is a different situation.:biggrin: