Narrowed 9" question

That isn't much back spacing for a 12" rim. OK if for drag racing but if you want to hit the twisties, you're better off trying to get the center of the hub nearer the center of the rim.

Nitto's tire chart's optimum rim width for a 315 is 10.5. 315s & 335s should fit fine on an 11" and keep the prices from getting outrageous for a custom rims.

You will probably do yourself a favor and find the rims you like first and have the housing and rear suspension designed to fit the rims. That way you can do some mock ups with the final product and get the spacing you want before you rip the car up, build it, only to find out your are off by 1/4".

$0.02

Check out Foose Design. Can't go wrong in his garage.