Sounds like drilling them out is what you'll have to do. Drill a small round pattern around the fastener head. There is a metal washer on the inside of those rubber plugs that expands the rubber to fill the freeze plug hole, so you'll be drilling through that as well...just so you're prepared. Too bad you can't pull the motor out...it would make it easier to remove them.
You'll have to pry out the remainder of those plugs after you've drilled through them.
Run water through the water jackets after you've removed all of the freeze plugs to wash out as much of the rust sediment as possible.
1970, 2WD, F-250, C/S, Dual Tanks, 390 FE (of course), C6, Power Steering, Power Disc Brakes, Dana 60 3.73, Sky View Blue, Ranger XLT
1970, 2WD, F-250, C/S, A/C, Dual Tanks, 390 FE (again, of course), C6, Power Steering, Power Disc Brakes, Dana 60 3.73, Wimbledon White & Sky View Blue, Ranger (almost twin brothers!)
"One of the greatest discoveries a man makes, one of his great surprises, is to find he can do what he was afraid he couldn't do" - Henry Ford