The irregularity in the gap at the top of the molding is from the wavy old-house ceiling. This will be hidden once the crown molding is up.
I am installing a two-piece crown, using a piece of baseboard upside down to trim the facing wall, and a crown molding installed over that. While I was standing at Home Depot, I could only decide on which baseboard to use, so I bought that and will choose a crown later this week.
In the meantime, I am reveling in this latest small accomplishment. The baseboard-crown-backer is up, and caulked, and the porch is looking a lot prettier ... little by little.