Dumb DEEP backspacing question

[laplander]

The truck is a 1967 with the 62" wide rear end with a short stepside bed on top with a 3.5/6 drop from ECE. Here's where the prolem lies. I've got a set of 17x8 American Racing AR 200s with 4" backspaing, that I would like to use on the kid's pro street truck. The problem that I am running into is I want to run a 29x18.5x17 tire in the back, but the most I can go as far as backspacing is going to be another 4.75 for a total of 12.75 inches of rim. At least that's what I'm figuring. Is that going to be enough, or do I need to come up with some other plan. To all you guys running deep backspacing to get the big tires out back, what were your measurments. Any help is appreciated, this is the last stop on the parts checklist. Is the 4" backspacing going to be ok on front with a 245/45/17? Or maybe a 245/55/17?
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