Will fuel pressure regulator stop engine from starting

It sounds like your car isn't getting the proper amount of gas. You say your mechanic changed the fuel pump and the regulator. Id have him run a diagnostic tool on it to see if it throws any other codes. Its getting fire. Try changing the FUEL FILTER. it maybe clogged. maybe the pressure regulator is not giving it enough pressure to push the gas into the engine or maybe the fuel pump is only working at times. Does it run good and then start to spit and spudder then die? if so its not getting gas some how. Idk if you can adjust the pressure regulator or not. i mostly work on old school cars.

What is the fuel pressure when the engine is running ?
Has the filter been replaced ?

The plugs should be replaced every 80,000 miles. Fuel filter every 25,000 miles.
Long cranking times with a cold engine are most likely a result of a defective checkvalve in the
fuel pump,causing pressure to bleed down at rest.Also recomend cleaning of injectors and air
intake system.

Sounds like the fuel filter. It would indicate the correct pressure at idle or maybe even revving. Once the engine is put under a load (highway speeds) it begins to choke. Try replacing the fuel filter.

Check the spark, remove the fuse to the fuel pump and remove one of the spark plugs. Connect the plug to its lead and position it well away from the engine against the truck frame (do not hold it) and turn the engine over, check the spark. If no spark it most probably a faulty coil. Check the impedance of the spark plug leads and replace any that are much higher than the rest. My guess is that you have spark but no fuel, What?... OK this is what I think is wrong. Your pump is fine when at position 2 on the ignition switch however when you switch to position three to fire the engine the power to the fuel pump is momentarily disconnected. A leaking fuel pressure regulator or injector would drop the pressure in the fuel line and at the same time flood the engine making it impossible to start. Once you start the engine with ether the ignition is back to position 2 and the pump can make good any loss in pressure but because the intake mix is rich this will cause the engine to run rich too. A good fuel system is capable of delivering 40 - 50PSI of fuel pressure and when switched off up to 90% of this pressure should be maintained for about 5 minutes, if it cannot check out the fuel pressure regulator and injectors. The fuel pipe should be capable of filling a pint pot in about 15 seconds, if it cannot check out the fuel filter is clogged.
Hope the above gives you some pointers

If it starts when you spray in starter fluid it would sound like a fuel supply problem. Make sure you are getting fuel and have a sufficient supply. Possibly a bad fuel pump or pressure regulator.

On that one it actually called circuit opening relay under left side of dash in fuse relay box

If the car fired after pouring gas into the engine, then you should check your fuel pump, listen when you turn on the key, if you hear the fuel pump start and stop, then check your fuel pressure, all fuel injected vehicles have fuel pressure regulators, if your fuel pump isnt pushing enough pressure to the regulator the car will not start, and if it does start it wont operate properly, and may even leave you stranded.. If your fuel pressure is low, check the fuel filter, dirty filters restrict pressure, and are ALOT cheeper to replace than a fuel pump!

Flel regulator will only affect max fuel pressure only will not cause it to not start. try replacing fuel filter first if still does not get fuel then you need to see if it is fuel related or if you have a spark at the spark plugs, pull off a spark plug boot and put screw driver in it hold 1/4 inch away from bare metal and crank engine if you get a spark then it's fuel relateed if no fuel it can be the crank shaft sensor or the cam shaft sensor. let me know if it has fire.

Check the fuel pressure at the shredder valve. You should have 60psi to 65psi with the fuel pump primed and engine not running. Once the engine has been started, there should be a 5psi drop. Make sure the fuel filter has been replaced and that the fuel pump is making at least 60psi. Anything less or more then 65psi will not activate the fuel injectors, the other thing to is that the spider injection system on the vortec are know to go bad and you may have to replace the system. Good luck and keep me posted, be glad to help you get your S-10 running 100% again.