Hmmmm.... if you want a "custom" dash then you may be the one supplying it! I really don't think there is one out exclusively for a Vette, let alone a 78-82.
I would think that you hav need to create a mold of your dash and then make one out of fiberglass. Or, just completely fab one out of 'glass. I have seen that done in everything from civics to 32 Fords. It looks cool- but you mat find it impossible to ever sell your car.
I would will my modified Corvette to my kids (that I don't have yet!!!) versus ever sell it. Oddly enough- my mom asked me once if I wanted to buried in it if something were to happen. I told her I'd rather she drove it and kept it up - as a 'tribute' of sorts to me.
As for custom dashes I think they work best as part of of an overall theme. I'd like to see a "20 year technology upgrade" modified Corvette. You know- something like an 82 with 02 wheels, engine, 6spd and custom fabbed 02 dash.
That would be killer.