I would check these guys out for sure:
- R+E Cycles
- Phone - (206)527-0360
- Email sales@rodcycle.com
- 5627 University Way NE
Seattle, WA 98105
They have a great facility. They're much smaller than Co-Motion, but are probably better equipped to get the fit right. They struck me as more experienced and analytical. Also, their prices, at least for the custom tandem they are going to build me, seem unexpectedly low.
Don't forget to get adequate heel clearance in whatever you have built. Considering how high your bottom bracket will be and the length of your cranks and feet, you're going to need to deal with that. I also have size 16 feet, but I use much shorter cranks and tour in road shoes, so I don't need as much clearance as you will.