is this the standard 55? 48" X 12" X 18"?
are you thinking of a 360 degree walk around tank? kinda hard with those dimensions.
the first thing you need to figure out is if the bottom is tempered or not. if tempered, it would be better to just get another tank. if not than proceed.
i have drilled many tanks, i have only used a diamond drill. they are expensive, but i trust this more than using a dremel bit. then again the dremel would be less expensive since you will prolly want at least 2 different sized holes. one for overflow and one for return. of course there could be more for a CLS, but it could get crowded under there.
G~