Hey, hows it going? I'm new to the forum and thought I'd reply to your post. Before I answer your question I'll say that I'm 6'0 tall (to give you a reference for size). When I went to buy a TT bike I was fitted for a 54 cm frame which is smaller than I'm used to riding. I was shocked but the guy fitting me said that TT bikes are meant to be smaller than your road bike. Once we got everything adjusted correctly it worked out and I have no problem riding 70 or 80 miles at a time. On the subject of the seat tube angle, I would strongly recomend you look at any cervelo TT bike. One of the nice features is the reversable seat tube, allowing you to have any seat tube angle between 74 and 79 degrees. I recently bought the Cervelo Dual and along with the variable seat tube geometry it's a blast to ride. Hope this helped.

Joe