Your numbers jive on my end. With a 255 I'd probably opt for the 9" wheel myself. It'll keep the tire from bulging outward. This tire is probably measured on either an 8.5 or 9" wheel.
Is there a reason you are wanting a 29" tire? You should verify you have enough room at the leading and trailing edges of the fenders and 1/4 panels for that tall of a wheel/tire setup. You may also want to estimate how much compression travel you have before you hit the inner wheel houses.
On 18" wheels I still typically recommend a 26" overall tire diameter for most of these cars, unless you really just need a bit more sidewall, then I'd go to a 27" tire. At 19" on the wheel I step up to a 27" tire to keep some sidewall on the tire. This typically puts you in corvette sizing where tires are easy to come by.
The 255/45/18 would be my recommended tire size for this car. You won't quite get 3/4's of an inch on both sides, but you'll still have enough room that I don't believe you'll have any rubbing issues.
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-Jason
1969 Pontiac Firebird
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