Great question.
I had a similar situation when I was getting my bike. It's an unusual brand around here, and although my lbs could order it, it's not a stock item.
What my lbs did was set up a bike that had similar geometry (but was a different material, had a longer top tube, had Campy components) to the bike I wanted, and I test rode that. Funny thing was, I hated that bike, but based on everything I told him, my mechanic could tell that the differences in the bike I was ordering would make it perfect for me. Waiting for my bike to be built was a bit of a nail-biter because I was nervous it wouldn't be comfortable. It ended up fitting like a glove.
I think you may have to test ride several bikes that don't quite fit, that aren't the exact model you're thinking of, and try to imagine what it would feel like in the right size/material. It's pretty much what you have to do when ordering a custom bike, too. Use the best lbs, one you trust, that will take the time to look at how your current bike is set up, find out what you like and dislike about it, and work with you to find something that meets your needs. Find out what the policy is if it doesn't feel right in the end. My lbs told me I'd be able to exchange my frame if it wasn't right.
Good luck!



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. I was out doing some investigating locally on how I could do a test ride.
They had received a new shipment of bikes that were all lined up shiny new and pretty in a row by the front door. Ahhh... the site and smell of carbon almost made me faint. Carbon does have some type of hidden smell to road cyclist, doesn't it? 
Any rate...
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