Actually mine is not threaded, it's press fit. Looks like I'll be pulling the engine again.

 

That is the welch plug in the back of the block, if thats not plugged you will have oil pouring out the back of the motor. As you have not stated that is going on I don't think thats your problem.

Edit to add pix

The top plug in the middle is the plug you are seeing from inside the motor.

 

The threaded welch plug is removed to access the press fit oil galley plug about 3" toward the front of the engine. The picture in the article I linked shows installation of a threaded oil galley plug. Mine looks to press fit like a freeze plug. If it threaded in, I think I could get it from the top.