If anything the road bike seat height should be the same or higher than the mountain bike.  Many drop their seat below optimal pedalling height on the mountain bike to make it easier to get their rear end behind the seat.  On the road bike it should be seat at the optimal height because you don't need to do that on a road bike.  I measure both the same way.  I set the seat height close to what I think it should be.  I sit on the bike and hold on to a bench.  I put the pedals at 12 and 6 position.  I extend my leg on the 6 oclock side and try to touch my heel to the pedal with a completely straight leg.  Make sure you have your bike shoes on and your heel should barely kiss the top of the pedal.  works well for me.   You probably feel like you are stretched out because the road bike has a much more agressive feel than a mountain bike.  It took me a few months before I wasn't afraid I was going to spontaneously fall over the handlebars when I first started road biking after mountain biking for many years.  It just takes some getting used to.  The 58cm size should be about right.  I could probably ride a 58cm and I'm only 6'.

I'm only 5'10" but my 54cm Cannondale felt a little small for me.  I actually rode a 56cm and it felt better.  I also ride a large Tallboy.  Comes down to preference and if you don't feel comfortable you should ride a 56cm to compare it to.  I would not want to feel cramped on a road bike though - especially on long sustained climbs.