Getting 27x11-15 drag slicks to tuck into bed sides?

[nikwho]

Hello all,
I'm trying to prep my '68 GMC for a informal grudge style drag race in Tucson in 17 days. My truck is lowered about 5.5" in the rear. I picked up a set of 27" x 11"-15 Goodyear drag slicks, in an attempt to get this thing to hook up. My bed is currently off, getting body work done. This brings me to my problem. I put the slicks up to the bed and they rub inside and out, which give me my obvious answer... Not buy smaller tires, but tub the bed. So, I'm going to remove the rear wheel tub's, cut the bed back, then widen the wheel tubs.

So, with my bed off, getting a little metal work done and ready for epoxy primer (bed is all bare metal now), I need to figure out my wheel backspacing. I'm wondering if anyone has experience to draw upon that can give me an answer, otherwise I'll just need to measure and hope for the best. Only looking to install a 15x10" wheel with a 27"x11" tire. I was leaning towards a 15x10" wheel with 5" of back spacing. I'll need to pull some more measurements, but does anyone have a similar setup, and what wheel backspacing are you running?

Thanks,
Nik