I know how you feel. I have the same problem with my stand being too small, but putting a 75GL tank in as a sump for the 55GL show tank just wrong

I am not saying you can not do it, I am just thinking that a 75GL show tank and 55 GL tank would be a better way to go. Also you do not need that BIG of a sump to keep all your stuff in it.
20GL sump for a 55GL tank would be good enough and it leave you with little room bihind the stand. Not much, but enough for a skimmer and auto top up outside the tank + all your other stuff could stay inside the sump