Plate at mast bottom removed, at last!
I finally removed the six screws holding the access hatch cover in the starboard side of the mast base. What a job! I tried penetrating oil, heating with a torch, hammering on the screws to free them, etc. but the only thing which worked at all was using a tool made from a star drill. A friend and I ground off two of the drill's 'flukes' to make a sort of an all metal screw driver of tempered steel with a blade which fit the screw heads tightly. One person tapped on the outer end of this tool with a hammer while the blade end was in a screw head and the other would twist it slowly with vice grips (a sort of a 'manual' hammer drill). It took about 30 min to remove each screw via this method. One screw was in so tightly that the head snapped off - and these are fairly large screws (their heads are ca. 1/2" across). The tapping was essential to the screws' removal. If we didn't tap continuously, the screw wouldn't budge. The plate into which they were threaded was so corroded that they were difficult to turn their entire way out. Not a fun job. However, thanks for your input...I have really appreciated all the advice!