You can't see an exhaust leak unless it smokes and that could be an oil leak. Hard to "feel" it because of the fan. If you're lucky it's an easy mechanical fix, exhaust leak, vibration (as mentioned) or normal noise. The louder the sound, the easier to find. If you're unlucky, just buy a used engine. One that you and a mechanic can listen to, check oil level, oil leaks, oil pressure,compression and maybe even drive it around under load to test it. You can buy 5-10 of those for $3K. If I were you I would buy a more modern engine. The AD guys here will tell you which ones fit. More power, better bearings, reliable. Chevy made some nice 6's in the mid 50's-60's and up. If you are a person who wants your truck to be totally original, buy a used one from that era. Denny says they last a long time so you have got a good chance to find a good one. A guy could to tear it down and then say you need a new block!, parts and labor are way high. Finding a guy who really knows what he is doing is not easy. Rebuild is sometimes a real PITA. For a total restore you need to have a "new" engine. You don't want to do all that work and have a used engine, But are you in that category? Buy a cherry picker and pull it out, very easy and mechanically straight forward. Keep all the parts that you can unbolt and $crap the rest. (IF someone says it's bearings or worn engine parts.)
