I am not tall — five nine and half (like noglider). So this is just a comment about fit and frames in general. I cannot answer to the challenges that are commensurate with very tall people. But here goes anyway:

My conclusion about frames and fit is that you will not know until you ride it. At very least you need to have a comprehensive understanding that is well and away from the idea of stand-over height as an outstandingly significant factor. And, all the dimensions can make sense on paper, but then ....

Here is an example: I best fit a 56 cm C/C (seat tube) frame. I have two of these, and the fit is totally different. On one of them, I have to nearly fall off the machine when I dismount or stop for traffic signals etc. On the other I need only a slight and easy tip, and then I'm at rest or off the machine. On one I need a high rise Nitto Technomic quill stem so that I don't feel that I'm in fight-to-the-death mode on a keirin bike. The other I'm using a very modest Cinelli stem that barely rises out of the steering head. By all good reason I should fit my 56-ST x 54 TT Simplon better than my 56 X 56 Vitus. But, it is the latter that actually is the better fit.

My take on this stuff is that the geometry of the frame is really important. You can do a lot with a mock-up frame to find out what you need. But those that I have been on cannot adjust for all the geometry. Great for choosing stems and dialing in for rough fit, but they only go so far.

I've wanted to replace my 54 cm Trek with a bigger frame. By measurements of inseam and height, I am supposed to fit the frame really quite well. I really squared up the math before I bid on an auction for it. Alas it is not the perfect fit. I have set it up so I can fit on it, but it is simply not the size that suits me best.

So, I have given serious thought about asking a local frame builder to copy the geometry of the Vitus and build me a steel version of the same thing. (His prices are very reasonable). The availability of suitable lugs may limit what I can have custom built, but that is a different topic — and I'm not flush with money. But it may be cheaper in the end to copy build a replica geometry of a classic that fits than to keep building on frames that are never really the ideal fit.

So my two bits is to buy the frame you have yer eyes on and see if it fits. I am thinking that eventually you will begin to "see" the geometry that fits you best. It may not be this bike, or your next bike. But eventually you may get there. In vintage machines, this is a challenging sort of process; we are juggling rarities in view of specific combinations of frame geometry along with respective body proportion.

Just for myself, and again just as an example, I am beginning to think that 56 cm bikes in road racing geometry with long head tubes that look like they are for a 60 cm bike are going to fit me better. It is just a mode of thinking I am in. Still, after diddling around with some vernier calipers, I have become more curious and more satisfied that I am right.

One thing to finish: when I was in my teens and 20's — even 30's ... anything I climbed aboard was a fit as long as I could adjust the seat to whatever height I needed. SIGH!