WTB tri bike (plus ? on fit)

I got into the sport about a year ago and purchased a specialized allez elite 58 cm.  I used a LBS and didn't really know what I was doing as far as fit, but the guy put me on a 58 and said it was right for me......so I agreed.  Anyway, I'm 6'3" 170 #'s with a long torso and have never really felt right on this bike.  I put some aero bars on it and used it through this season, but I am starting to think that I need a larger frame for my body size.  I have been doing a lot of research online and went to a few LBS around Dallas trying to get a feel for prices and what type of bike I want.  I have been told that I would need a 61 cm on a road bike, but would need to size down to about a 58 cm on a tri bike.  The problem is that I can only afford a used tri bike and the LBS aren't very receptive to me getting locked down on the proper frame size b/c I guess they can tell that I am really not interested in buying new from them.  I feel lost as to whether to go with a 58 or 61.

So, if anyone is looking to sell a tri bike frame 58 or 61, I would love get some info from them.  Also, if someone can share some knowledge about the best way to figure out what frame size to go with based on my body style.  Any help will be appreciated.