Tar-like undercoating on 63 Valiant...any way to remove it?

I've used several methods over the years, all with varying levels of success. I will often use a combination of different tools to get it all off.

An air chisel with the pressure turned WAY down works really well on flat spots. You just have to be careful to keep the tool at an angle so you get underneath the coating without beating the crap out of the panel. It should come off in strips if done correctly. You can also use an oscillating scraping tool.

Another way is to heat up a paint scraper to cherry red and scrape it off. I personally don't like the fumes when doing this unless you have a lot of air flow.

Last method is a stiff wire wheel. It gets everywhere. Super messy job. This is my least favorite way, but sometimes you need to do it in tight spots.