Jeff brings up a good way ahead and definitely the safest. The only thing I would add is to pull the freeze plugs and dig out the gunk that hides there, especially the lower ones. I did not find a lot by removing the head but that was my experience,others have. So from what I read it comes down to choice. If you want it drivable now then I would ask if your truck is over heating in the traditional way. By that I mean is it dumping antifreeze on the ground when it runs for an extended amount of time. From what I read that may not be the case. If not you may just need a thermostat and as stated before, these are not pressurized systems and they have to find there own level. So the reason the overflow tube was plugged may be because they did not know what to expect. Is this way ahead ideal? Not really you may or may not be delaying the work. However It may get you enough to drive it the rest of the summer or while you tackle a few other areas or even look at going deeper. Just throwing out another option. Which in this case means my 2 cents prior to inflation.