are you sure the 56 lht is too big for u? i'm 5'10" and use a 58! we may have different body types, but also it could be you are used to a smallish fit. i think a larger fit suits a tourer like the lht better.
frame size fitting can be a bit of a religious subject; i think you just gotta figure out what works best for u. i've transitioned from a small, racey fit to a larger, comfy fit and have been happy so far.
here's a read on it from the rivendell dudes:
http://www.rivbike.com/article/bike_...g_a_frame_size
as far as top tube clearance, things do get a little snug for me if i stand-over it with both feet flat on the ground, but I find in practice its not an issue.
if i got a 54 or below, my question would be if its hard to find thinner 26" tires. like the equivalent of a 32 or 35 on a 700c. This is the size i like to use on my lht.