do you have the old starter if so take and have rebuilt or take in to parts supplier and they can match it up. A GM dealer would also be able to match numbers of the old one to new. some times all there is different between many models is Bendix tooth count or shims between block and starter

the 326 was not offered in 1968 for the firebird. i dont know if that helps but if you could get the engine code off of it that would be helpful. it is stamped on the right front side of the block or it can be found beneath the production engine number on the machined pad on the right hand bank of engine block.

Sources: The Standard Catalog of Pontiac