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.