Well, I ended up taking my heads to Bearing Service Co. in downtown NW Portland. They've been in business since the 20s and have a really cool, very large, old shop. More importantly, the guys that work there were very knowledgable about the Mitsubishi V6 heads (all the different models) and knew about the eccentricities of the design.

I'm having them pressure test the coolant passages to check for cracks or pinholes, do a three angle valve job, check the guides and replace any that are out of spec, install new seals, and resurface the heads. Should be getting them back in a few days.