I suggest you do the "bottle test" to check the FPRV--[Fuel Pressure Relief Valve] that is the sensor on the fuel rail. LLY engines do have a weak spring in the FPRV.
On the drivers side fuel rail near the firewall there is a horse shoe shaped 1/4" hose close to the glow plug bracket. Disconnect the hose where it comes out of the fuel rail. Then hook up long hose and plug the "U" shaped hose.
Run the hose to somewhere that a bottle can be mounted. If your hose is long enough, you can mount it between the coolant tank, and airbox. Anywhere you can have the bottle standing in a upright position.
Now drive the truck hard, with some wide open throttle runs.
After you do this, check the bottle. If its empty, your FPRV is not bleeding off fuel and you don't need to shim the valve.
If it has more than a little fuel, that means that your fuel rail relief valve is leaking and you will then need to either shim the valve, or install a race valve.
Chevy 04.5 LLY, 2X, shell, XDR, MikeL Heavy duty up-grade, 6spd, TransGo, deep pan, de-badged, 65 Gallon replacement Aero tank, LBZ-mp, Kennedy pump, shimmed, Pre-fuel filter, tow mirrors, BU cam, gauges, GPS, Sirius, bags, custom grille, fender vents, HD trans cooler- Curt front hitch, footwell lights, load lights, Studs, head gaskets.