I don't think there's anything to really worry much about. Oil will thin out when it gets hot, which will lower the pressure reading. Most likely, judging from what you said about this being the longest trip taken in it, that you haven't really driven very far with your truck. You may have never got it hot enough before to notice the lower pressure for the oil being that hot.
For a bit of explaination on the pre-'54 engines, they tend to run at no more than 16 psi or so when hot. They are low pressure engine, using dip and splash oiling to the rods, pressure to to the cam and rockers only.
