A little more progress.
Because I'm running undercar exhaust I applied a thermal ceramic heat coating (from Second Skin) to the underside of the floor boards, trans tunnel and differential area floor panels. I rolled it on with a foam roller. It' a heavy paste but I guess an undercoat gun would spray it. I doubt it would come out much smoother or uniform from an undercoat gun however. I've decided to leave the aluminum panels out for the body prep work. I may even wait to install after paint. I'm not sure of when ERA does this in their shop - before paint or after?
I ground down the mold seams with a dremel tool and sanding drums after wiping the body down with Acetone and 3M scuff pads.
Initial pass with sanding boards and blocks on the body to show high and low areas. Many of the low areas are very shallow and may come out with more boarding. I think I'll leave the low areas alone for now and work it a panel at a time - scuffing up the low areas just prior to skimming with a thin coat of lightweight filler to help me avoid losing track of where they are and where I am. Some of the deeper areas around the panel openings, I'll probably use a fiberglass reinforced filler initially to flush them out.