My guess on your curved rear windows is they're an acrylic material, aka Plexi-Glas most likely given the era of the body. The bad news there is they can't easily be polished back to an ultra clear appearance. It might be best to contact a specialized plastic fabricator in your area and get their input.
I'm looking forward to your progress for this whole project, drive train, interior and exterior too.
Had the crew in the movie The Fantastic Voyage
worked the streets they'd have driven one of these!
Insulation wise I'd seriously look to use foam panels, up to 2" thick if possible everywhere they can be easily fit into place. The floor looks to be an excellent place to begin. Side walls might require a bit more creative cutting and shaping but the end result of excellent thermal transfer resistance would be well worth the effort.
Any dimensions for the interior as it is now, empty?