I thought I felt the world stop!!

OK - it sounds like opinion time(this ALWAYS gets me in trouble)

Why Eastwood when your local paint supply has some good epoxy for the rear and other chassis parts? I happen to like SPI, but DuPont/Nason make good epoxy and it can be tinted or you can use the flatted DuPont black over it. My thing with Eastwood is that it isn't local, it's a premium price, particularly when you need to add shipping and many of their tools and products are also available at Harbor Freight for a lot less, and are exactly the same.

I would probably leave that seam between the roof and main body unless there was major rust pitting or a rust out. It may have some sort of webbing and will need to be removed before you can weld and finish fill. (I just looked at a bunch of my pictures of '37s - and of course most all had that seam filled )

The door bottoms - I think that I would redo them with proper patch panels. Most '37s doors hang open at the bottom corner anyhow and the added thickness just exaggerates it. I looked at probably over a dozen at York a couple years ago after someone mentioned it to me in conversation and with one or two exceptions, they all had that gap. Here's a photo of the EMS inner and outer panels: http://www.emsautomotive.com/Pn107.html.

Whatever you do, I'm sure it will look great!!