My truck turns over great but does not start at times?

You need to start with the basics, The engine needs three things to run, fuel, spark, compression. As it was running okay and then started this compression probably is not the problem.
GM 4.3s have a couple of common weaknesses. Bad crankshaft sensors, bad ignition modules and fuel injector issues.

I would start by seeing what is not there when it will not start, lack of spark is easy, just toss a spark tester on it and watch for spark. No spark = test the ignition module and crank sensor.

Next get a fuel pressure gauge and test the fuel pressure. Many parts stores have tool loan/rental so you dono t need to buy the test gear.
https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator
Pump failure is common on GMs once they get some miles on them. If it is a bad pump the easiest way to change it is to remove the truck bed and work from the top.
Do not shop by price on a replacement pump, the OEM pumps are generally better than the after market units.