Holy Cow. I remember when you couldn't GIVE a Britbike away... Wow. Of course if you have had that '74 for as long as I'm guessing you might have, you probably have a LOT of history together. Which would affect things. From a purely financial standpoint it sounds like a pretty fair deal, however.
Personally, I wish I would have kept all of the 13 or so bikes I've owned over the years. But man, a '06 sporty even up...
Good problem to have I guess. Not much help here, sorry.