How do I remove big flathead screws in the main??

[Makr0]

Hey Guys

I have a weird problem from another post. My gooseneck broke but I realized I have two from where a previous owner moved the boom.

I want move this old gooseneck to where my current broken one is. Problem is the flat head bolts holding it to the mast are really on there. I’ve used penetrating oil, left it for a week, reapplied etc.

When I try to use my impact drill to remove the screws the drill just slips out. It’s driving me nuts. Man I really hate flat heads.

Any tips on working with Flatheads??