That's exactly what I did. Standing over the two bikes, the 58cm was too tall but had good reach and the 56cm was perfect, only not quite long enough. I got a 120mm Ritchey WCS stem from
BikeSmart (they had the best price), and I couldn't be happier. Changing stem lengths is common practice in fitting a road bike. I'm a little over 6'0 tall but have these neanderthal arms with a finger tip to finger tip reach of a little over 6'3", I'd have to change the stem length on pretty much any brand bike I'd buy.
One suggestion I will make is don't make the same mistake I did and try and fit the bike by moving the seat back to gain reach. I did this for the first 400 miles and developed some really sore knees. Invest in a longer stem, you'll be much happier.