Maybe the "plate" you are referring to is the rear crank seal carrier that bolts onto the back of the block. The seal carrier is aluminum and forms a large part of the oil pan sealing surface on the rear of the engine. Depending on where he ground it, you may need to replace the piece.
It would really be good to find that leak somehow before you re-do the LIM & UIM. That is, unless you are planning on pulling the engine anyway. But, if you find the place where it is leaking first, it may not be necessary to pull the engine.
I was thinking maybe you could isolate the engine cooling system from the radiator and heater core (so as not to damage them) by removing the hoses and installing some pvc caps as plugs. These could be secured with hose clamps, then one of the engine heater hose nipples could be connected to a garden hose with another clamp to pressurize the engine cooling passages. Under pressure, it may be much easier to find the leak, especially since it is apparently so big.
Lastly, since you have already bought the Dorman and not the APN upper, you might want to consider pressing a ring onto the reduced diameter Dorman EGR stovepipe (so it will fit into your '95) and installing a shield around the pipe to protect the plastic of the upper from heat deterioration. If you want to try this, let me know.