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

Started pulling the old fuel line, and, as I worked my way forward, realized the whole underside is coated in a dry tar-like material. This 1963 dealer rustproofing, a 70s JC Whitney thing, or what?

Any way to get it at least mostly off without blowing the car apart? I fear it'd turn into another unfinished project if I did that.

Thanks,
Steve