Yeah the 66-67 rear wheel space is VERY limited, backspacing IS the key!
heres everything I've pulled off the net and various forums since I'm trying to decide how big a wheel and tire will fit under my 67 b-body (plymouth) hope its helps!
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I run Weld Rodlite 8" with 4.5"BS on the back. I've used BFG T/A 275/50-15's, BFG Drag Radials 275/50-15's and Hoosier QTP's 26x11.5"-15. You can't go any wider than 11.25" section width without changing your backspacing or moving the fender well lip out a little. Alternatively you, can move the spring hanger and run more backspacing or even better run a modified shortened differential housing.
However, for street trim 10.5" is plenty of tire width. Mine is a 3.55 ratio so I stuck with 26" tires. I'll be going to a 29" tire for racing in the very near future and consequently going to a 4.10 or 4.30 depending on engine demands.
Here is my experience with wheel/tire fit on my 66 Belv. (At stock ride height)
I bought my car with 15x8 Cragar SST's and 235/60's on it. The backspace was 4.25" and the tires would rub on the stock sized quarter panel wheel lip. Not always, just during moderate speed turning, or if I had passengers in the back seat.
Then I picked up some 275/60's and some 15X7 cop rims. The cop rim backspacing is 4.5" and the narrower rim along with the slightly more backspacing allowed me to fit the fatter tires with more clearance than the first setup. I had about 1/2" between tire/wheel lip and about 1/4" between tire/leafspring. Looked very nice from behind. Good luck.
My buddies 67 GTX has a Goodyear 28x9 slick on a 7" rim. I think the BS is 4 1/2" but I can check with him to make sure.
After some careful measuring, we found out that about a 275-60 on 4.5" BS was about the max.Tried a buddies pair of 295-60,8" rim W/4.5 BS,they cleared the springs by 1.5" and were flush with the lip of the wheel opening.
I run 28x9x15 M/T slicks on a 15x8 Cragar Dragstar with 4.5 backspace. This fits in the wheel well with no problems, but getting it into place will require you to raise the car so the rear drops as much as possible. I had 3" wheel studs in my axles because of hte rim design, and with those and the low wheel lip, I couldn't get the sidewall bulge past them without either deflating the tire, or disconnecting the shocks.
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