An old mechanic thought me a trick many years ago. If you don't have an impact driver, take the biggest screw driver you have, set it in the slot, and as you hit it with a BFH, twist the screw driver like you are tightening the screw. (this also applies to impact drivers)
The way a screw or bolt has its streignth is on the thread edge facing the head. This edge may get rusted and won't budge. By tightening the screw/bolt, it will break the rust bond and you should be able to remove the screw. I know it doesn't make sense to tighten a screw to remove it but it does work.
This is exactly how my M715 brakes worked out. The drum screws had not been removed for many, many years. My 14" flat head screw driver and a 1lb sledge hammer was all that was required.
Check this out.
Wheel Cylinder Conversion
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