[bigblockv6]

The 305 in my 68 had a very similar scenario driving down the freeway, it locked up so I shoved it in neutral and pulled it to the side. It was overheated somewhat and low on motor oil. Once it cooled off I was able to fire it up and there was some knocking going on, It wasn't too upsetting since I was building up a 478M V6 for it. After pulling the rod bearing caps one of the rod bearing was eaten up but surprisingly nut spun. Your case he bottom end is seized and a rebuild would be in order. You may want to consider just finding a good used V6, there has been talk just this passed week on someone that has a collection of GMC parts and V6 engines you may want to look into.