18x10's on 68 Camaro, willing to shorten axles!

[Erik Beckett]

I am looking at a set of rear 18x10 rims for my 68 camaro project. They have a 4.5" backspacing, so I know they will not fit stock. My plan is to mini tub and shorten the 12 bolt anyways. Does anybody see any potential issue here before I fork out the cash? Is there standard math that will tell me how much I need to shorten the rear axle? Its the stock 12 bolt from a 68. i plan on running a Glink.

Thanks, Erik