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About the only thing you can do to convert the residual red rust and to remove the mineral deposits from years ago, would be to use at least two different acid treatments.
I have cleaned out blocks and heads, typically on the tractor before I split them, so I can get the pipes and all cleaned at one time.
The first, I would use Muriatic acid, which will dissolve the minerals and most of the red rust. I'd rinse it well, then fill with the straight out of the jug Phosphoric Acid, which will phosphate flash the cast iron, as well as convert any lingering red rust into inert black oxide.
If you were really detailed, you could epoxy coat the inside, but with all the blind passages, you'd almost have to fill it with liquid epoxy, then invert it to drain and cure, and that would take a lot of very expensive epoxy primer.
Personally, I would acid treat it, and it should be much cleaner for the next 50 years, but I doubt that you will notice any difference. You'll need to do the head as well, and then you are left with the radiator tanks and core, but remember, the core won't tolerate the strong Muriatic acid wash.