Greetings from the dry side of the state.
Look on you '60, on mine there is a plate that cover a flat spot in the floor panel on the sloped surface towards the door.
Can't say if the plate will be there on your '66 but should see a flat spot where the factory mount hrdro-vac air filter wlouold mount. I beleive Ive got the filter set-up off a '59 C60 that would work . Unless you require the o.e. look you could run the hose to a filter anywhere you want.
BTW, nice fleet!
