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.
Bob
1937 Champion D2C Deluxe Lite Twin
1954 Johnson CD-11
1955 Johnson QD-16
1957 Evinrude Fastwin 18
1957 Evinrude 3022
1958 Johnson QD-19
1958 Johnson FD-12
1959 Johnson QD-20
1982 Evinrude 25hp
“Every 20 minute job is only a broken bolt away from a 3-day project.”
"Every time you remove a broken or seized bolt an angel gets his wings."