What color should the bottom of my 66 be? semigloss black or the car color?

Which assembly plant? Not all plants used the same primers. You should do some investigation under your car to find out what is already there. Do not expect the entire bottom to be painted the car color, just overspray around the edges. Same thing on the floor inside the car.

Kansas car.

Kansas cars, like my 66 El Camino, were black underneath. The outside of car was primered with gray primer that oversprayed the bottom and then body color which should cover most or all of the primer leaving most of the bottom black. Don't forget black undercoat in the rear wheelwells.

Don't forget black undercoat in the rear wheelwells.

Were the rear wheelwells on all 66's undercoated from the factory?