Drop-out spacing is the issue. I adapted a Shimano HF-08 9 spd hub to my 7 speed Santana Visa. I tried removing the spacers equally and it brought the small cog too close to the drop-out...and the Santana has 140 mm spacing! I, instead, cold-set the spacing to 145 and it works perfectly. I think that 145 mm would be pushing your old frame too far. Pushing your bike to even 140 is a stretch. The chain line might also become a big problem. I don't think that it is worth contemplating. I would sell the bike for whatever I could get, and go for a newer tandem. If you can find even an older bike set up with 8 speed, you could easily upgrade to 9 speed. But, I would think about a late '90's 9 speed. I've seen many for mid teens. There are other advantages...go with 700c wheels and your choice of tires increases dramatically.