How
should it be painted? However you are happy with it.
The factory did not specifically paint the underside. At the factory, they shot paint at the rocker panel, and some of it oversprayed onto the bottom of the car. So most cars have paint extending about a foot or so under the car. (My 2006 Chrysler is the same way!)
Having said that, most of us are building these cars to drive, and want the added protection and good looks of paint under the car. (I realize this is one glaring inaccuracy in my resto, but I want it to look nice!)
If you are going for accuracy, I would suggest duplicating the factory job by not painting under there. But I would make sure to have some good protection of the undercarriage (epoxy primer or something...)
-Tom