The 1969 production number for Sprint Convertibles is 267; still digging for 1968 numbers, though I found one reference that claims the breakdown for 1968 does not exist. It might be contained in the 1967-1969 Firebird restoration guide, which I do not have.
When I ordered the PHS for my 1967 Sprint Convertible, the production breakdown was part of the package. Perhaps someone else has run their '68 VIN through PHS and might be able to provide some info. I have a 1967 and 1969 - no '68 yet.