You could go as wide as 14 inches in the back and 11 inches in the front. Historically correct would be 15-inch diameters. But the big wall you're going to hit is tire availability. If you get the 9x11x15's you're screwed. In fact, in any diameter, you're going to find very few tires wider than 335, which will fit on a 12-inch rim.
So, depending on if this is a track-only car or a street-track combo, look at tire selection before you have rims made.
My car has the earlier RSR sizes (9- and 11-inch flares), and I run 9.5x11x17 wheels with 255/315 tires. I couldn't do this with 15- or 16-inch rims. Your car will have an extra two inches of width, front and rear.
Gratuitous picture of my car:
Another Pelican guy recently has flares made in the widths you're talking about. He'll be running 18-inch slicks on the track. I think he'll need to run narrower wheels on the street. If you do a search on the word "Mirage" you can find his thread about making them. The thread title will have "RSR+" in the title.