Erm. I have a 42 or 46 cm surly pacer with a less than 500 cm top tube that I'd trade for that rawland's frame
http://www.surlybikes.com/pacer.html
YOu probably would do well on the 46 cm surly pacer that's got about a 500 mm top tube. The 50 cm has a 520 mm top tube which might still be fine on you. The rawlands you have has a 560 mm top tube - so that's 40 more & then another 40 that you have on the stem.
The surly has a steeper seat tube angle than the rawlands as well - so the seat'll be closer to the handlebars in general. The rawland's is fairly slack at 73 degrees.
but my suggestion would be for you to test ride some surlys and see how they fit.