Z06Hulk

I'm hoping you guys can help me here. I have an opportunity to buy some wheels that I really like, and couldn't find the answers I looking for very easily.

I have a set of CC wide body fenders on order, so I've begun my wheel search.
The news wheels in consideration are:
20x13 rear with +63
19x10.5 front with +57

Tires are already mounted:
345/25/20
285/30/19

The offset calculators I've been looking at show a 36mm less inner clearance, and an extended 28mm outer position (compared to stock on the rear), but doesn't account for a 345 tire either.

I'm stumped Been reading post after post, and just can't find the answers I'm looking for. I looked at several offset calculators:
http://www.corvettecentral.com/wheel-offset/
http://www.willtheyfit.com

And still can't find real world answers.
I really appreciate any help you guys can give me on this, as I really like these wheels and would love to make them work with the wide rear fenders on my car.

Thanks so much
Rocky

Z06Hulk

I got the backspacing measurements:
Approx 9-3/16 for the rear
Approx 7-3/4 for the front

I measured the current setup and I show I can go a max of about 8-3/4 in the rear. So if I ran a 1/2" spacer - would it still work with a wide body kit?