I talked a friend out of an early 70s Swan 40 or 41 just a few months ago. It was an S&S design - the older style, tumblehome hull and a deckhouse, not the quasi flush deck style that came later.
It suffered some serious structural and cosmetic issues and still was listed for over $100K.
I wasn't aware of the tankage issue, that's a biggie, and found the accomodations less than impressive (esp by later standards) and some of the berths positively claustrophobic.
Based on that viewing I think they rely a lot on the name, and that they, as much as any boat, need a good hard look and serious survey. Also likely to be overpriced.
You do need to very careful with the really old ones like the 36, 41, 43 or 55. Like a lovely classic car they can suck you in and convince you that any shortcomings can be easily overcome. In your dreams that is.
Having done the exercise and having nearly made a very expensive mistake I'm now much more likely to go for a younger boat, pay more up front, but have less hassles down the road. (Down the Main ?) The boat I considered buying is still on the market over 18 months later , same price but supposedly they have thrown $30.000 at it since then.
The old Swans suffer from interior comparision in particular, although only if your idea of the perfect yacht interior is the likes of Benneteau etc. Me , I prefer the old style althought with the Swan the amount of work to bring one up to scratch for liveaboard cruising is, to my mind, not worth the trouble. If what you wanted was a lovely old boat for pottering the bays and harbours with not much offshore then it may be a different story.