That's not completely true.
The Offy was intended to be a exhaust heated intake. I have prepared many of them for water heat and run one myself. Simply pull the steel staked plugs out of the bottom and weld aluminum plugs to seals the holes. Make or buy a plate to seal the bottom and run water thru it.
The oldest cliffords had either No heat or used exhaust heat, the more modern Cliffords are designed for true water passage flow. Although, I have heard complaints of some leaking.
