Originally Posted by Diseasel300

The 3.0L 603 had head issues, solved with a #17 or later head casting and is an extremely durable engine. The 3.5L 603's had quite a few problems with bent rods, ovalled cylinders, headgasket breaches dumping oil in the cylinders, etc. Supposedly later revisions solved the bent rods/ovalled cylinder issue, but it is still a relatively fragile engine compared to the earlier 3.0L version. Some say that if the engine has >100K miles on it, it's probably good to go.

With any 603, keep an eye on the temperature gauge like a hawk and make sure the cooling system is kept full. If you have a leak, fix it. They don't tolerate being overheated well. Kept maintained, they're pretty bulletproof.