Unless you want to purchase a tool from Snap-On called the crud buster and want to spend $200+, then I'd suggest a 4 1/2" angle grinder and two or three loosely-bound wire brushes. (they don't produce hand fatigue or as much heat as the spiral wire brushes do.) Then after most of that is off, I'd suggest a couple rags soaked in Acetone to rub the last of it off. I think mine took me maybe 4-6 hours total to do the underbody and inside of my '73. (on the old seam sealer.) Though I didn't use this method, I'd say a putty knife and a small propane or MAP gas torch could do the job, too.
