I kinda feel like a broken record on this one. A Vortec small block (with the original poppet valve injector setup) fuel pressure spec is 58-62 psi. Once they start getting down around the mid 50's, this will vary from truck to truck, there's not enough pressure to open the poppet valves and the truck will not start. If the pressure is marginal you may get it to start by cycling the key enough times for the system to prime and the pressure to just barely crack the "good" mark, it may run OK but stall unexpectedly, etc.
Chasing other scenarios is kind of pointless until the fuel pressure issue is dealt with. If the filter is old, try replacing that first.
Those with the injector upgrade to the MPFI setup find that their pressure runs consistently a little lower than spec, that is fine; those are real injectors with no poppet valve setup and they are far less finicky about fuel pressure.
Richard