Ircanuck, I used the 34mm RVC extension. As Josh said, it's not so much a problem with the valve extender, the 51mm stem is just too long. The interface between the valve stem and the extender is almost at the rim. You shouldn't have any issues on your 808's. The real problem is the lack of latex tubes with shorter valve stems and a removable valve core. Perhaps SILCA could get into making latex tubes???

Here are the latex RVC tubes I know of (clue me in on any others):
Challenge - 47.5mm
Vittoria - 51mm
Bontrager - 51mm
Vredestein - 52mm