Took my radiator off for the first time in 20 plus years. The bottom outlet is really getting eaten by rust. I have only run water in it. Leave the water in during driving season. Drain it for winter. Why? And what can be done to slow or stop it? I can only imagine the inside of engine.
Straight water with iron parts on the Model T will do only one thing, create iron oxide. The formation creates rust damage.
Yes, the block and head will rust, nothing lasts for ever. To retard rust always add to plain water an anti-rust agent. For best care, use distilled water or even ionized water as pond, tap, or rain water has minerals that cause deposits and can start corrosion too.
Anti-rust products include water soluble oils, and modern anti-freeze. Use either.
Anti-freeze will lose its rust preventive as it ages, so to care the best for that old iron block, head, iron parts on the radiator, and the radiator brass core; drain, flush, and refill with 50-50 mix of distilled water and anti-freeze annually.