The 3 coolant holes (approx. 3/4" diameter) are rough cast, have a draft angle, and unreliable position. Good luck with a close fitting dowel. The head bolt holes are a different story...I digitized several heads to determine exact position of the head bolt holes in order to make a torque plate. Position was within .005" on all of them so I consider them a reliable datum. If I wanted a jig I would bore 4 headbolt holes, drill the 6 angled holes and then gas nitride it rather than install drill bushings. Jigs with crude datums give crude results.
