The Parts Index offers only replacement part information. While a copy of the '67, '68 or '69 Parts Index would be an excellent resource for the corresponding year car, it would not take into account running model year changes such as dash vent balls etc.

The later (mine is '73) copies of the Parts Index offer replacement part information that often reflects revised or reissued parts. Sometimes the part numbers have been altered from the original, but sometimes the part number is the same and the part changes in some way that is not deemed sufficient to create a new listing. So the Parts Index in itself is insufficient documentation.

The A.M.A. specifications for the model year were published prior to model release. That allowed for service manual providers and other publications to analyze and compare data. The A.M.A. specifications also do not have 100% accurate information as to the date of manufacture.

Assembly manuals have some excellent data, including part or process change dates. However, I am not aware of any '67 or '68 Firebird assembly manuals in circulation.

Dealer service and sales bulletins are an excellent resource. But it is necessary to store them sequentially and keep complete sets as one can supercede another. Other than this site, I do not have access to a set of service bulletins for '68, and do not have '67 or '69.

And the final item is physical inspection. I have several sets of heads, some rebuilt, many original, from 350s and 400s. All have dual valve springs. I can't fathom someone taking a set of stock 350 2 bbl heads with press in studs and going through the trouble of installing dual valve springs without making other changes. And the existence of the dual valve springs is supported by the A.M.A. specifications.

There are of course still unexplained mysteries, such as why my '71 YS block is factory drilled and tapped for 4 bolt center caps. Or why a block I sold, from a 400 2bbl Bonneville, also was drilled and tapped for 4 bolt center caps. None of the "regular" 400s were ever delivered with 4 bolt caps. GM would not have drilled and tapped those blocks if they were not for potential use in a vehicle that would require them. After all, machining operations cost money.

"Bird Glass" has been bandied about as a huge mystery. It's no mystery. It took a bit of cooperative input from members here with and without 'bird glass and the answer was quickly found. Documenting every detail of every unmolested car would greatly increase our knowledgebase. Kurt S. and the CRG group have done an excellent job, there is no reason that we could not research, review and publish a comparable set of information for Firebirds. That is why I took the added time and expense of plastic media blasting over any other type on the '68 restoration, it revealed previously hidden writing on the firewall that I would never have known existed otherwise.

Correct fan on a Ram Air...why was the four blade fan standard equipment? Did it perhaps have to do with stiffness and stability at high rpm, as it was the same fan specified for the OHC-6? The standard 400 non A/C fan was a 5 blade flex fan. At high rpm, it flattens out. Unfortunately the reasons for the selection are not given, but the A.M.A. specs give the usage.