You have probably finished the tar removal but this MIGHT (? ) help someone else maybe, well maybe not , well not at all. The heck with it I'm not posting (just kidding).

My '56 Canadian Dodge 2 door wagon (Plodge) has a similar tar on the floor boards under the seats (lower right in pix) and I had a air powered gasket scarper air chisel that I have not used much that worked very well.