rear wheel fitment

I have a 65 Pontiac Tempest, same body as a GTO, for those not familiar. What is the best way to determine the proper wheel offset to give me more options for the rear tires? I currently run
P215/65R15's on 15 X 7 rims with a 3 3/4" backset and without Airlifts in the rear coils, the tire scuffs the inner fender. I would like to run at least P245/60R15's on a 7 1/2 or 8" rim.

I can't return new wheels once I have mount tires on them. Does anyone know of a foolproof method?

Here's a link to a great tool to have around the shop, it's made by Percy's and when used correctly is right-on accurate!!!!!

http://www.summitracing.com/parts/PHP-01201/

Lots of places other then just Summit sell it....

The device Dave suggested is handy, before we could get those we used a tape measure or rule. Here's a guide from the Wheel Vintiques folks that shows you how.

Thanks, guys!