Oh man, you're going to convert a 550 gallon tank into a sump?! I'd just look for a used 200-300 gallon tank for that. The 550 seems like it would be a waste to use as a sump because that is a much rarer size tank. It would certainly be sufficient though. You could grab a used 220 and a used 125 and make sumps for both the 755 and the 550, and then run both.
Another option would be to sell the 550 for a decent price and then buy a used 200-300 gallon tank much cheaper for the sump, and then put the saved money towards some nice fish.
Any way you go about it though, I'm sure it will be awesome!