I have no first hand experience but I would seriously consider Glyptal as the only choice for painting the inside of the block. It is proven to work in an environment where failure will be catastrophic to the health of your engine.
Paint chips floating around in the engine, getting on bearing surfaces, clogging
oil pump pickups, blocking lifter oiling, etc is no way to find out if Rust-o-leum is better than Glyptal.
Glyptal is available in aerosol cans and brush-on cans. It doesn't take much.
Here's a pic.
John