Here is the skinny. I have a 70 Formula 400 I'm trying to put wheels on but I need to know the correct backspacing. I've heard everything from 4" to as much as 6" (the latter I highly doubt). I would like put 275's out back using a 9.5" wide wheel. Is this possible to do without rubbing or sticking out past the fender?

The weird thing is that most shops can tell you the front backspace (4") but no one can be 100% sure about the rear. I've see articles in HPP magazine with a couple of 70-72 Trans Ams with 245/45/17 and a 4" backspace in front and 275/40/17 with a 5" backspace in back. My question is, where there any modifications needed to fit this setup in the rear?

Here is the wheel and tire combo I'd like to use:

Front 18x8 with 245/45/18 and 4" backspace (Total diameter approx 26.7")
Dunlop SP sport 9000 - width approx 9.6" req wheel (8" wide)

Rear 18x9.5 with 275/40/18 and 5" backspace (Total diam approx 26.7")
Dunlop SP Sport 9000 - width approx 10.8" req wheel 9.5" wide

My current setup is 16x8 235/60/16 all (no rubbing or anything)
Width is about 9 1/4" (total diameter approx 26.6)

I measurred the backspace of the stock 14x7 wheel and it's almost 4.5". If there is someone out there running a setup that is close to what I'm looking for please post pics. Also, let me know the true backspace for the 70-73.  

Birdcatcher,
Sounds like you've done your homework- some of the older T/A rims, like the aluminum 15 x 8 'snowflakes', had 4-4.5" backspacing (I'd have to dig out one out of the garage to confirm). I've even seen some G2 'Birds with 275/40R17's on 17 x 9s front and rear, but the owners weren't telling if that package rubbed in front- looked sweet, though.

Your proposed setup looks okay, but your tire height seems a bit too tall- I'd stay at about 26" max. I had 255/60R15s (27") on 15 x 8.5 rims (4.25" bksp.) and they would rub the rear fenderlips when the suspension was at full compression. I went to 235/60R15s and the clearance was good. It would still be a good idea to roll the lip on your rear quarters to give those 275s room.

Want to hear something crazy? I want to run 315/30R18s on 18 x 10.5s- obviously cannot be done with the stock chassis, but there are plenty of aftermarket fabricators selling narrowed bolt-in front subframes with 14” Wilwoods, coil-overs, tubular A-arms, rack and pinion steering. The back can be done with a mini-tubbing and new relocated frame rails (you go 4-link at that point).

Geno