To answer your question on pistons and gaskets, Funky, stock head gaskets can be used with pistons up to 77.5 mm. in diameter. (Note to readers: The preceding is based on experience. If you wish to call attention to lack of same, go ahead and contradict it.) Re. valve guide seals, the only seal other than the stem seal is the O-ring at the base of the guide, which can't be seen without removing the guide from the head. I've never seen one of those fail and I can assure you that even leaving the O-rings out entirely wouldn't cause nearly the kind of oil consumption you've experienced.
Anyway, while you're into the motor be sure to check guide to valve stem clearance; I've seen quite a few that were worn out. When there's too much slop between valve stem and guide, the stem seals get wallowed out and fail prematurely.
Anyway, while you're into the motor be sure to check guide to valve stem clearance; I've seen quite a few that were worn out. When there's too much slop between valve stem and guide, the stem seals get wallowed out and fail prematurely.