Trailmix, nice bike and one you can enjoy with the changes you are mentioning. As others have said, I'd get it ride-able and give it a try then you will know how far to go with your build.
I'm not a fan of how the brakes are currently configured so that would be something to consider with either drop or flat bars configuration. One fun thing, you might be able to use your rear shifter for your drag brake if you change to drop bars and bar-end shifters.
It will be interesting to hear what the spacing of the rear triangle is. If 140 you still have opportunities with some of the tandem specialty shops. Tandems East has 140mm spaced wheels and hubs so they could probably get you set-up with a free-hub set-up for not to much. If you guys enjoy riding the bike, which is your first hurdler, moving to a 7 or 8 speed set-up would be pretty inexpensive, something like Shimano Tourney for example if you were to go new.
It is a pretty bike and one that can be ridden and enjoy.