Finally, a copy of the Mercury Service Manual arrived last night.

AWWWW! had to go and spoil all the guessin' fun by goin to the book!
Well good for you! At least you got to where you wanted to be. As for
the hogged out hole, this looks like an opportunity for "JBWeld to step
up to the plate. Clean the daylights out of that hole, I mean REALLY
clean it! Mix up enough of the A part, and B part to fill up the entire
threaded hole. (from your description it goes entirely thru the
casting?) if so that is great. Working from the BACK side of the
casting will guide you. After the JB sets drill in the EXACT CENTER
with a small pilot drill, 1/8 or smaller, then when satisfied you have
a good center, drill with the tap drill taking all the time you need. A
drill press would be handy for the previous steps. Hand tap then,
backing out very often, and try to pick up the original threads.
Carefully run your tap thru. As an afterthought somewhere I have LO
temp ALUMINUM stick, that I tried to use to plug a tank hole
unsuccesfully. Did melt the stick, but I did not have enough heat to
have it fuse to the sheet. Was using a propane torch. With your
casating you might have a similar problem. If there are better ways to
recover from the situation, we will soon hear about them. maybe
another member has a better recommendation

Den 48tfYF