I would at least give Santana a call. If you want it "KOF" but also want some "modern features" why not at least consider a company that has probably made more tandems than all other builders combined. They have more experience with SS couplers and have many more tandem specific tube set choices, etc... They used to routinely make fillet brazed frames with post construction sculpting. You are going to end up with tons of money tied up in this project so I would at a minimum explore all options. Frankly for the same dough you could probably buy two tandems: a used modern 26 inch tandem and a real vintage one (my 1st choice!!).