due ruote 

quemazon

I've read that using a 145mm hub on a 140mm steel frame works OK. Have you considered that?

conspiratemus1

Tandem chain stays are very stiff. (Our old Santana uses fork blades.) Unlikely would you be able to cram a wheel in there that is 5 mm too big, especially with horizontal dropouts where you have to drag the wheel backwards too.

The horizontal dropouts are perhaps why the previous owner used a nutted rear axle. The quick release may not have clamped enough to keep the right end of the axle from slipping forward with two people pedaling hard. Yes, and adjust the eccentric -- (IF it's not seized in situ, you know, like an aluminum stem or seatpost can be ) -- to snug up that timing chain. If it falls off you'll likely crash, sort of like when a crank breaks. Seriously, with the chain that loose, the bike is unride-able junk until/unless you can move that eccentric.

Other than that, I'd ride this bike totally as is, unless there is something that's clearly broken/worn out and needs replacing. There is nothing to indicate that there is anything wrong with with freewheel -- it's just old and is perhaps a dead-end design. Freewheels can be very difficult to remove from tandems because they get driven on so hard in low gear. So just ride it. If it's noisy or sticky drip in thin oil until it behaves. If you don't want to take the hub apart (say, because you have to remove the freewheel to get at the right-side axle nuts), just drip oil in there too and be done with it. The need for grease is way overblown.

Suggest you insert a long M5 bolt through that hole in the fork crown (where a caliper brake would go) and secure it with a nut so that the free end of the bolt sticks just far enough forward to catch the straddle cable, should the main cable ever break, and keep it from snagging the tire. This was a greater hazard with knobby mountain-bike tires, admittedly. But it's a simple measure that gives you one less thing to worry about.

If you have a new baby (Congratulations!!), I can't see the two of you getting a lot of saddle time together for a while yet... So spend it riding, not faffing around trying to upgrade it or fix what ain't broken.