I would think of it as a starter tandem and not try to 'update' beyond what's needed to get it rideable. I bought a 1980 Sovereign new and faced a similar dilemma as it aged.
When a new 'stoker' came into my life a few years ago, we decided to ride it for a while until something better showed up on Craigslist. It wasn't a long wait. We got a Cannondale that fit us much better, had 9 speed index shifting, etc, for $1000. Though it had 700c wheels, it still had the 140mm rear end. But that was a problem for another day.
27 1/4" wheels will not be a major problem, but as noted earlier, tire choices are limited. Of course, sizing is the first consideration. After that, I wouldn't consider buying another tandem that didn't have 145mm rear end, 9 speed index and threadless headset. All of that can be had from any decent tandem built after 2000.
Then again, a mid 80's tandem might have Phil hubs and BB's and an Arai drum brake, which in themselves are very desirable by vintage tandem aficionados.