A few thoughts:
1) 135 is MTB standard, so no problem there, except for the drum brake threading, but 135mm tandem freehubs do exist.
2) If it's a freewheel hub, 7 speeds are the most you're gonna get. The only 8 speed freewheels I'm aware of were the Sachs ARIS. (And I think I remember hearing about a Regina, but have never seen one.) ARIS freewheels were good, but they haven't been made since the late 1990s, I think, and the 8 speed ones are basically unobtanium.
3) Shimano 8 speed shifters index just fine on 7 speed clusters. I've done this many times, using 8-speed bar-end shifters. You just have an extra click. Put it on the high side.
(Fun fact: The opposite way also works. I ran 7 speed thumbshifters on an 8 speed mountain bike, because I hate trigger shifters. Shimano thumbshifters have an overshift "below" the 1st click, and since the inner position is set by the limit screw, you just set them up so that the 1st click is on the second cog, and it just works.)
4) If you're really unlucky, you have a tandem-specific 6 speed freehub, which only very-hard-to-find 6 speed Uniglide cassettes will fit. Supposedly, you can make Hyperglide casettes work by filling out the one skinny slot. Saint Sheldon has a write-up, and it's supposed to work, but I've never had to to try it.
Bottom line, there are several good ways to remove the hide from this feline.
--Shannon