You'll want to get the actual (rated) dead load rating for any shelf you want to buy and make sure it can handle ~2,400 lbs in the manner you want it to sit. No one can estimate it's capabilities (even knowing it's six feet by 2 feet) without seeing the type and thickness of the metal and fasteners and its structure. Any shelf properly designed for that load will have dead load ratings available.
Yes, it's possible there might be such shelves there, but if you use one that is overloaded you risk serious issues.
Any replacement or modification of any significant load bearing portion of a structure should be regarded as speculative in nature. Imo, one should use shelves that are rated properly "as is." I've personally seen alterations that were "reasoned" to be better by building contractors (in my professional experience) that led to disastrous collapses.