I belive the only difference between a LS and an LSX is the latter isnt an in-car productions motor. Its a platform crate engine....cast iron...with the potential of huge displacements with the use of long stroke cranks. If your wanting a monster....go with the LSX block from GM Performance....think they have them priced way down right now. If 550 hp is what you'd concider a mid performer....then IMO a drop out from a F-body would fit the bill perfectly.
http://en.wikipedia.org/wiki/GM_LS_engine
BTW. Thanks Pat for the hint on the Corvette donut mounts. Those will look real nice polished up.
Dave G