Coolant leaking in the back 350 chev what would it be?

What specifically do you mean by "in the back"? If it's leaking down the back of the engine it could be the intake manifold, head gasket, heater hose connection, heater hose or rusted freeze plugs. But you can't know until you inspect for the source of the leak. That means getting right down there, maybe under the vehicle and have a bright light with you. If it's a coolant leak there should be a buildup of colored coolant deposits that leads right to the leak.

Good luck.