It ultimately being up to the GM is just the beginning; I need solid guidelines lest I start 'doing my own thing', leaving players in the dust with every free-wheeling turn I take. My favorite adventure is A Kringle in Time, which blithely trips through time and space, changes genres with breath taking impertinence, and prefers funny to logical.
BTW. The 'stress rules' are in a free pdf magazine, Uncounted Worlds #2, at http://www.chaosium.com/. I also found two arctic adventures that will fit in with an Tunguska Event arc. (In Weird Menace: Supernatural, the Tunguska Event is caused by someone opening a Templar fortress of solitude incorrectly).