Wheel fitment diagram for 70 Chevelle Help

[SS454]

I had some time on my hands this morning so I made a diagram of the measurements I have for my rear wheels. My question is after looking at the numbers, does anyone think I could squeeze a 12.5 or 13" wide wheel in there. Backspacing numbers from the experts would be appreciated, I want the tire as close to the fender lip as possible.
Also what does the brake caliper spacing have to do with ordering, is that going to affect the offset?
I also posted this on the Chevelle web-site but I thought you Guy's were more informed on 345/335 series tire than that site.
Finalizing my choice in wheels and finishes is the hardest decision I've made on this car, I don't think a day has gone by where I did'nt change my mind.