I wouldn't get too aggresive with the prying on those points.
That aluminum is very soft and a screwdriver or other tool will make horrible gouges.
I would continue whacking with a rubber mallet, then if you could get even the slightest crack to appear, try some wooden or plastic wedges. And the heat may help, who knows.
If it does have that old Permatex, it's truly horrible stuff, it basically turns to epoxy after a few years. Of course, it may just be bare gasket, you never know.....
You're experiencing a really common problem with these old bikes - that gasket has been there for a very long time.
In the past it's always been the "shock" value of the rubber hammer that's ultimately broken them free, for me at least.
Your problems are only just beginning, as once you have it broken free, you'll have to scrape the old gasket off, which will literally take hours and hours.
You may in fact end up just leaving the old gasket on there and applying a judicious (meaning small) amount of silicon.
It's always a toss-up between the benefits of a new gasket and the potential damage that could be done (which could be significant) in scraping the old one off........