I got all 4 stuck screws out with lots of heat, penetrating oil and patience.   I didn’t have to drill or tap new holes but whenever I remove any bolts during a restoration project, I always run a thread chaser down each hole just to clean up and remove any corrosion, salt residue or other crud.   Here’s a pic of my 4 screws after I got them all out – the screw at the far right and the one at the bottom in the picture came out with vise grips, the other 2 at the top I used a double jam nut.   I think I would try applying some torch heat on your 3 stuck screws before using the impact driver.    If they are super stuck, the impact driver might just snap them off.

One should always use a tap to chase those threads to ensure they are as clean as can be. Also note, being that those screws are not very big, your impact driver may not do what you hope, penetrating oil would be your friend and some heat to get it to soak in there.. Patience cannot be overstated here..

The challenge is the screw head kind of seal the top so getting solvent in there is a challenge. If the head breaks off, try to remove al lthe others, any that break, THAT is when the solvent can work its way down the broken screw to get into the block…Again, patience…

OK, getting lost in this thread?  Did the poster ever get the powerhead freed up?