[Pete]

With your situation, (new rebuild and good radiator) and all possible checks reveal nothing that helps, quite often the block was ONLY hot tanked or shake and baked, NOT acid stripped.
Nothing will remove the rust and scale except acid stripping. Usually the most build up is around the base of the rear 2 cylinders.
These engines could run stone cold (figuratively speaking) from the factory.
They should do the same after rebuild. (if done right)