You could try a bottle of head gasket sealer which goes in the coolant. Although I have no experience with these sealers they don't seem to have a very high success rate. I would personally never use these sealers, I would just tear it down and replace the gaskets. The only time I would personally use a sealer like that is if the car was my dd and was running really bad. Then I might use the treatment just to try to make the car usable until I could fix it the right way. So imo just replace the head gaskets and only use the sealer if you need to get another few months of driving out of the truck before the repair.