I need to install a new heater control valve in the 1963 sedan I use as a daily driver.

So I thought it would be a good time to rehab the heater box I took out of my red 1964 sedan years ago. I got it taken apart by unscrewing lots of short sheet metal screws and saw a couple of curious things.

There is a mostly dry white goop between the two halves of the heater box. Is that the butyl rubber that is between body panels on other places in the car?

Also, around the heater box is wrapped some insulation (see photo) that looks like the asbestos-looking stuff that is pasted to the floorboards. What would be a good replacement insulation wrap?

I forced water through the pipes in the heater core and cleaned the fins carefully with a wire brush. Is there more I should do with the core before I put it back together?

As for the rest of the heater box body, I plan to sandblast it and give it a nice satin black inside and out. How do I remove the rubber bottons before sandblasting?

Thanks for your wisdom!