if you are patient and good at crossreferencing, you can find all the original specs on http://www.1968firebird.org/index.html

you might get 2 or 3 options.
the original parts will only be a rough guide anyway, for optimum tuning of your specific engine and climate. I dont know if the difference in fuel from then and now make a difference in tuning, but I bet it does.

I found the original specs, and this article:
http://mywebpages.comcast.net/gtopercy/Pictures/QuadrajetTech.html
very helpful in getting mine close.

edit: 1968firebird.org seems to be down. hopefully it will come back up. there are a couple mirrors of it in different places...
here's one: http://209.237.45.104/index.html