Don't be surprised if the nipples on the block are partially clogged. Just clean them out and you're good to go.
Yeah, I was thinking this is possible with corrosion at the metal nipples, the water pump and t'stat housings are already kinda cruddy... if the crap gets dislodged and then caught in the core.. and it won't show up in the radiator cuz it's all stuck in the core..
..today I'm waiting for my local gas station to finish a State Safety Inspection on it before I get further into this..
Cheers, - Art