I have a '58 D17 gas, and my brother has a '59. . both are great tractors. about 54hp stock, but overbore sleeve kits, a AC 175 gov spring and cam, and you can really get a lot more power out of the engine.
Many of the cracks seen in heads are because people didn't allow the Diesel to cool before shutting down. Do a pressure test, if it doesn't leak, don't worry about it. Common with AC heads from what I have read & seen.