Several years ago I purchased a copy (decent reprint) of the 1972 Pontiac Parts book, Illustrations and parts groups which includes the parts for the '68 Firebird. I suspect it is the same one Yellowbird had on her site for many years. It was massive, consisting of several hundred pages. I finally took a day to take out all the non-1st Gen pages and got it down to three binders. I got it from an auto manual vendor.

There is no known 1968 Firebird Assembly Manual available. The 1968 Camaro and 1969 Firebird Assembly Manuals are as close as you'll get.

That teufert.net parts book that barnbird found is the best I know of available online. It appears to be a '76 version which might explain the slightly different format than my '72 manual version.