worth rebuilding a chevy caprice engine?

Water pump.

With that unit your anti freeze leak is probably running onto your timing cover from either your water pump (not likely) or your intake manifold or if luck is really on your side, the thermostat housing or hose. Clean everything off to be dry then run the engine till you see where it's REALLY coming from. Seen alot of these aluminum intakes leak at the water jackets (corners). Theres is no way anti freeze is leaking from the timing cover on that particular engine. Also if your that close to passing emissions, throw a set of plugs in it, check all the wires & cap/rotor & replace them if needed. This situation usually doesn't require rebuilding the engine over unless it's got mucho miles or you know it's a long time coming. Good luck.

Like others have mentioned, if you have antifreeze running from the timing cover, you have bigger problems than a leak. Only oil should be behind your timing cover. If you indeed had a coolant leak from the timing cover, it would not be green, it would be a chocolate milk color from water mixing with oil. Chances are that either you intake gasket is leaking and the water is running to the front of the engine (not as likely) or a leaking water pump gasket/gaskets. If the water pump itself was leaking you'd see the the water leaking from the weep hole on the shaft of the pump. If it's coming from behind, it is most likely the water pump to block gaskets.

-Bruce

If you want to pass emissions, change your oil and plugs.

Yes, rather obvious water pump. It's $15 and about an hour.