At 5' 7.5" I have ridden 54 c-c bike for the last 30 years. In today's measuring systems that is about half way to a 56. Yes, proportions are different, but 54 is generally considered the sweet spot for 5' 8". Subject to adjustment for long or short legs, arms, torso, of course. Perhaps all OP needs to make the 54 or 56 even just right is the stem flipped up until he gets accustomed to road bike positioning. Or order the bike with the fork uncut, and start out with extra spacers until he is accustomed to the position. Most of us know that he could be fit just as well on either size with minor adjustments. He just needs a shop amenable to that.