"Given that this is a once in a lifetime sort of investment" I feel the same as Gary Jay, absolutely try'em before you commit. KHS makes a Lenord Zinn designed bike for tall people that comes with 190s, maybe see if you can ride one of these even if its only on rollers in their store. Even then the short time you ride them would'nt give you the full story. Were talking 20mm(.8") difference, so the bb would be that much higher for the same clearance on a bike made for 190s. I think the dialog for very long cranks(over 180s) is open for discussion as far as benefits go, Zinn's advertising talks about them like the best thing since sliced bread, other folks not so much(i'm in the latter category). I have a 39" inseam(yep, pretty tall guy) and I have ridden 175s or 180s for over 40yrs. At this stage I wouldn't consider going for a set over 180s but that's just me. If you could borrow some 29er(700c mtb) wheels with a taller, fatter profile to fit your current frame it would raise your bb about that 20mm so you could see what a shorter drop would do to the bikes handling. Another thought; you probably know someone who rides 180s, see if you could put them on your bike and give it a try. Maybe have the frame designed for 180s(which is how my frames are designed) which is a compromise, but it would be a shame to discover 190s are not for you after the fact and end up with a bike that's not right for you.

Good luck,

Brian