The effective top tube length of my 51 cm Leader 725 is 3 cm longer than my 54 cm SS roadie project frame and I'm 5'8" as well, they are long and low. The max I would have gone at our heights would have been a 52 cm frame. I think if you use shallow/compact drops with your current stem it might bring your arms in at a more comfortable range. Too bad the fork has already been cut and slammed.
It's much easier dealing with a frame that's too small rather than one that is too big.