Kansas' later lps are pretty poor it has to be said: but the first self-titled LP is a gem IMO, and perhaps the nearest any US band came to replicating the European Prog model.
"Leftoverture" also has its charms, I s'pose, aswelll. the live version from Two for the Show is killer!
Another act that came to mind was DUST - they're Hard Attack lp especially - but that is more of a Rock-with-progressive-tendencies type of thing than a PROG Lp proper - but no the worse for it. Quite a Wishbone Ash feel to it!