Thanks!
Thanks to all for the excellent advice! I gave up and took it to the bike shop. The young man had it off in half a jiffy. He had the Park tool, which I may buy one of. But I think I also might have loosened it for him. ;-)
And I remember now that back in the shop we always applied some grease on the threads when we installed pedals to help prevent just this sort of thing. The action of pedaling works to tighten the pedals on the crank. So there is no danger of them coming off. And when it is time to remove them, it goes easier.
Anyway, now I can remove the 165 cranks and install the 171s and reuse the pedals.
Thanks again!
Saul