The 06/07 SEUC's are indeed standard Ultra Classic suspension. If I'm not mistaken, it is slightly over 3" of total travel. the only down side to lowering both ends is removing 1" of travel. I'm also 5'8", 29" inseam, but elected to go with the Legend Air Ride...I can drop the rear of the bike 3.25 inches when I need both feet flat on the ground, then raise it to whatever height I want for ride/conditions while moving. Platinum Air Ride is another good system. They are pricey, but far and away superior shocks to the stock units. But if you're after a better shock, IMO before I spent the money on the shorter stock HD Air shocks, I would at least go with a shorter version of the Progressive 440's...a lot better shock than the HD stock unit and you've accomplished the same goal. The stock suspension on any HD leaves a lot to be desired...you don't really know how bad it is until you've ridden something that's been improved. It all depends on your ultimate goal...just getting your feet on the ground better, or improving the suspension at the same time.
There is more than one way to skin this cat, depending on what you want to do and how much money you want to spend, as always.