Hey 80vette
Try this link
http://wiki.corvettefaq.com/index.php?title=Interior
Item Lines 11, 12 and 16 have alternative options in the form of C4, C5 and after market sets into a C3.
Another option?
The L81 electric seat floor pan is deeper than the manual one.
I have a similar problem with height (actually with width as well, but that another storey!

)
I removed my electric seat and replaced it with a manual version, thereby reducing the height of the lower seat cushion and gaining about 1 1/2 inches in head height.
Although more radical, if you already have manual seats (not sure if 80 had electric?), you can get more head height replacing with the electric floor pan from an L81 or 82 vette. Not cheap, but would work.
Just my $0.02 worth, good luck with what ever you decide.

Grant