the oil passage comes up vertical from the pump/rear cap then makes a 90 deg turn out horizontle to the filter/thru the filter then back in horizontle into the block & back to the same vertical passage and a 90 deg turn back into it & the oil continues further up vertical to the horizontle oil gallery. The vertical passage between the 2 horizontle passages is drilled so the oil would go strait up without going out to the filter and it needs a mini freeze plug driven in to a depth between these 2 passages. There's a cupped depression sort of there for it to seat at the right depth between the 2 horizontle passages. under the rear main cap are 4 holes, 2 for the main cap bolts, one for the intermediate shaft that drives the pump and one for the long vertical passage and that one is where you drive the mini plug in. EDIT to drive the old plug out so you can bristle brush clean all the galleries drive a rod into the oil psi sending unit hole above at the top rear of the block to drive the plug out the bottom aforementioned hole