If you can find one which clamps to the chain stay and to the downtube I think you should go with that one, as they can be moved around a fair amount so you shouldnt have too much trouble adjusting to your frames geometry, rather than using the really long type which clamps to the seat stay by the rear cog and your downtube. I think your best bet would be to use one from the 70s or 80s that came off a 3 speed or single speed.