[gmc684x4]

The return less rail also suggests its a 05 motor
03 truck motors are a return style fuel rail

The only for sure way i know to id 4.8 vs 5.3 is pull the pan and look at the rods and crank

Real good chance your motor is an 05 4.8 ls motor
They are still a decent motor I had one in my service van ran great
The pcm should also have a vin in it for what 05 vehicle it came from

Powerblock had this happen they bought a 5.3 from the yard pushed it hard on the dyno 1000 ish hp then opened it up and discovered they got a 4.8 instead