After reading over the last few posts again, I made a point of checking out the Spyder scraper attachments at a local home center. Brought a coupe of 2 inchers home with me, and headed out into the garage to see how they'd do.
Like anything, there's a learning curve. First run was straight down the flat, raised area of the passenger floor from just between the seat brackets forward. Using the scraper blade in a variable speed Craftsman C3 cordless recip, I tore through the tar in about 5 seconds until I hit a change in floor topography about 4 inches back from the firewall, at which point the saw bucked back quite nicely

. Rethink the process, and started revving up slowly using the low speed to start. I learned to "start" the scraper by using a front corner to nibble at the "tar, then move the blade slowly back and forth. Between the repeated impacts, and the slight flexing of the floor, the tar came up pretty quickly and completely. Still have some hand chisel work to do, especially near the rubber grommets and the narrow channel, but I was pleased with the results of less than 15 minutes of work.
So, I'd recommend the Spyder 2 inch; not sure the others (4 and 6 inch) have much utility with a car resto, but I'm keeping the wider blades for the next home project. Once the chiseling is done, wire brush everything, epoxy a couple of pinholes, then POR, primer and paint.