I've moved on.
Block sanding now. On advice of the painter who showed up to look at the car (guy's doing a SEMA car now; says he's booked solid through July—can't wait for his estimate!) I'm blocking with 180 before moving back to 220 and spot-priming before wet-sanding.
Put about 10 hours in this weekend. Guide-coated with some black lacquer which is not ideal but so far so good. The car isn't going to need a lot more fill.
Given the responses (aka non-responses) I've got so far from the painters I contacted, I'm resigned to the idea that, most likely, I'll be shooting this thing myself.
Toward that end I ordered a couple of ceiling mounted 1500-watt electric heaters. With my little oil filled, plus the mouse-eared propane unit behind a couple of 22-inch box fans and two layers of plastic sheet on the open end I should be able to raise the temp inside my back bay high enough, with low enough humidity, to work with the paints I have. Obviously I'll start by 2k-priming the whole car again after the blocking's done.