About 50 years ago, I was able to seal a cracked block on a 1947 Pontiac I owned with a product called K & W seal. It held the year I owned the car and I saw the car on the street 2 years later and I don’t think that the engine had been changed. I would never use the product with a car that had a lot of value, but I had paid $75 for the Pontiac and didn’t want to put a lot of money into it. The crack was around one of the valve seats. The valves were in the block because it was a flathead engine. I thought the problem was a head gasket, so I removed the cylinder head and then discovered the problem. I think I paid less than $5 for the can of sealer.