Yep, after market wheels are the problem indeed. I'm my younger days I was a big fan of the Cragar SS wheels. They use to manufacture them as a direct bolt app to the make (GM, Ford or Mopar) of car. But now they have "one size (wheel) fits all". I had friends get problems with these. I was lucky to have the older SS wheels, no problems.
Getting the Rally’s with the correct offset will help I'm sure. Personaly I'd stay away from aftermarket. But if that's the look for you, make sure the wheel will fit without the aid of spacers.
Below is what I posted in another thread here at FGF; Size of Cragar SS Wheels: Christine, yellowbird posted this site to clarify sizing rims. The second site is for tire size comparison.
The important things to consider are the backspace on the rim and tire size. 15" rims are ok to use, but keep in mind not getting a tire to tall or wide. With the '67-8 'birds the fender and tub size are less forgiving for bigger and taller tires than the '69. Also, if you’re lowering the car this will limit you further.
I would stay with the same backspace as your stock rims. If you want to go to a wider tire and rim you will want a deeper offset. As to how much, other '67-8 owners will have to key in. With wider tires wider rims are needed to avoid sidewall bulge. Generally an inch wider rim is required for a wider tire.
In my case, I'm running P225/70R14 tires on stock 14x7 Rally II's on my cammer. These tires have a diameter of 26.4". My cammer isn't lowered; and I've experienced fender rubbing on steep driveway aprons and ramps with the wheels turned hard. I wouldn't put this size tire on my lowered convertible.
IMO a tire size with a 25.5" - 26" (aprox) diameter will be a safe bet being your going with a lower stance. This would calculate out to a P215/70 R14 or a P195/70R15. With a wider tire perhaps a P245/60R14 or a P215/60R15. But being a '67-8 you may have problems with wider tires in the front. If you are going to mix tire size from front to back, try to keep the tire diameter close.
I hope I haven't given you info overload. Play around with the web links and you'll get a better picture of what I tried to convey.
There are differences between the Firebird and Camaro springs. Mostly it’s spring rate. I don't have the specs, but I'm pretty sure the ride height was close. So if you have a V8, the Camaro springs you have now are close to what was factory installed.
I installed progressive rear springs on a '72 Chevelle I use to own many moons ago. As I recall, there was no significant change in the ride regardless of condition. But they sure helped when I was towing a trailer.
