Welcome aboard.
We have an entire subsection of the forum devoted to hydrogen and HHO found here:
HHO and Hydrogen - Gas Savers - Fuel Efficiency Forum
You may want to note that not a lot of people post in that subforum. HHO is not that popular around here as it really isn't proven.
I have honestly thought that a corvette with a small engine would do really well given that the frontal area is so small and it is relatively low to the ground and light (lots of fiberglass). I would think that with the right motor (1-1.7 liter I4) would make it hit 50ish mpg. There again, corvette owners would be screaming "BLASPHEMY". I always thought it was a cool idea.
We'd love to see some pics of the car.