When I did mine, it had factory underseal about a third of the way across the floor from the sill lip inwards. The central third, the transmission / prop shaft tunnel was actually preserved by years of oil leaking & once de-greased revealed the original clean painted floor, no need for heavy duty prep other than usual prep for painting.
For most of it, warming with a heat gun (don't try & melt it, if its smoking its too hot) & follow it with a sharp chisel gets most of it off.
After that I got a 3 inch dia wire wheel on a regular mains powered hand drill & rubbed the remainder & the fiddly bits it off that way. No need for the heat gun when using the wire wheel, best done cold.
Change the direction of the drill every so often as the wire wheels go "handed" after a while, so change the direction on the drill & you will feel it "grip" / become more abrasive again & repeat