Yeah, there’s a number of possibilities. I agree looking at fuel pressure is a good place to start. A problem here brings the fuel pump, it’s relay, the fuel filter, fuel lines and even the tank, etc., into play. And ok fuel pressure pretty much eliminates most of these, as well. You said you replaced the regulator, and the pump was fine. I know it’s probably not the case, but I have to ask - any chance you were low on gas when the truck troubles began? Could you be out of gas? If not, have you tried starting the truck using a starting fluid?
The next thing I’d go to is spark. See if you’re getting spark at the cylinders (plug wire boots). If not, start tracing backwards from the cylinder toward the distributor. If you are getting spark at the cylinders, then check the ignition coils, and spark plugs. You could have a bad rotor button, a bad distributor cap (even moisture inside the cap could create problems), bad plug wires, ignition coils, and/or spark plugs.
These are not the only possibilities making your problem, just my thoughts on how one might address these few things, and either find a/the problem or start eliminating things.
Hope you’re able to find the culprit.