OK here's what you'll need to do. Pull the oil pan, oil pump, and the #5 main bearing cap. Measuring from the main cap mounting face up into the oil pump outlet hole on the passenger side of the block should only be between 1 1/2" and 1 3/4". Any more and it's either not there or pressed to far in.
Some blocks were machined (years ago) with out the step that would stop the plug so it got pressed in to far.
This plug separates the oil filter feed from the oil filter return.