You can run 275/50R-17s on an 8" rim. That's what I have on mine. You can either run Falken's, Nitto's or M&H Drag radials in this size. My backspacing is either 4.5" or 4.75" but my rear is about .75" - 1" narrower than stock.
235/40R-17's on the front.
The 9.5" will fit and some folks have run 285's on them.
http://www.highperformancepontiac.co...tiac_firebird/
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1967 Firebird ragtop, 464, Weiand 8-71, ported 6H heads, Hyd roller, 4L80E, paddle shifter, Moser 9", Cal-Tracs, Vintage Air, Corbeau seats, 13"/12" disk brakes, Hydraboost, electric cutouts and M&H drag radials. 
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