Cbeitel,
I think the 55 will work out fine, but I've thought of a few more things that might affect your decision.
All of them revolve around the clearance for getting into the sump to service things when it is in place. You stated the stand is 43" tall. I assume that is from floor to the top of the plywood you plan to sit the show tank on. The working space inside the stand will be considerably less. Looks like you would loose 2" from the floor to the top of the plywood you plan on setting the sump on. Looks like you'd also loose 4" from the top. If the sump tank is 21" tall that would leave you 16" of working space to get things in/out of the sump. That might not be a problem, but it might be if you plan on placing a skimmer in the sump. Also, consider how well you will be able to access things in the furthest corner of the sump. My stubby little forearm is only 17" from elbow to finger tip. I'd have problems reaching the bottom and back corners of the sump.
Another consideration is the width of the 55 gallon - 13". Again you might want to consider skimmer dimensions.
Sorry if these seem like silly things to consider. You might want to stick your arm in the 55 and see how well you can reach things on the bottom and back corners. Hate to say it, but I'd probably sacrifice some volume and go with a 30 gallon breeder (36" long, 18" wide, 13" tall), or build something out of acrylic.
Sincerely,
-Sci