Any tips for removing gear housing oil drain screw?

1997 240 Sundeck with 5.7L EFI Alpha Gen 2

I am having no progress in removing the drain screw from the lower housing to replace gear oil. The vent screw did come out reluctantly -- it had no washer seal. The lower screw is merely deforming. I have soaked it many times with penetrating oil (even overnight twice), but the slot just deforms and the screw does not move even a smidgen. Tried huge blades, rapid tapping with smaller screwdriver on the outer flanges. The screw slot just keeps deforming with no movement. It must come out. And of course I do not want to crack the cast housing body. All tips (or new cuss words) gratefully accepted.

I have already replaced the impeller. In removing the lower unit, I found much corrosion on all the bolts. Until I bought this low hours boat from an older couple, the boat had been maintained by a well known boatyard near their waterfront restricted community. It appears it has not had honest service, just quicky service. In draining the onboard gear oil reservoir, the bottom half inch was gelled dark green oil perhaps resulting in no oil feed to the lower unit. I have cleaned this out. What oil I could get from lower unit by holding it upside down is dark but I want it completely drained and flushed before refill.

Any additional tips in removing this screw without causing damage are certainly welcome.

If a hand impact didn't do it then you have 3 choices. have someone weld a 1/4 SS bolt to it or drill and tap it or heat it , ad hit with some Freon computer cleaner The sudden hot and clod might work.

I'll try to locate someone in area who can weld a stud to outer surface when businesses open tomorrow. I do not know whether this screw has a magnetic inner pin for collecting metallic debris or not, so I am afraid to drill the screw. I certainly do not want to pop a magnetic pin loose in the chamber of gears.

definitely not recommended, have welded numerous one, they come right out

Thanks again Bt Doctur, for your suggestions. I located a welder (about 10 miles) who attached a bolt and removed the drain plug. It was still very hard to break loose. No indication a gasket had been installed, and the plug did have the magnetic pin. Very glad I did not drill! Even so the welding heat had loosened the pin so it fell off in my hand as the plug came out. Located replacement and spare gaskets on way home. Thorough flushing, installation, and filling to follow. Should have boat back in water tomorrow if the infernal rain will stop.