Vortec 5000 heads appeared only on light trucks from 1996-’97 L31 engines. They had raised Vortec ports with 1.94"/1.50 "inch valves. Earlier heads had the same ports but had smaller 1.84" inch intake and 1.50" inch exhaust valves. They both share the same 0.470" lift at the valve restrictions as the 350 Vortec heads. These heads will net you 26 horsepower over a fuelie head.
They do require their own self guided rockers that are different than the stock SBC parts (so if you have to waste money on a roller rocker that will save you 4-7 horsepower in parasitic drag, also known as friction, buy the self guided roller rockers). The horsepower discrepancy in parasitic drag varies as a result of open spring pressure. As the spring rate goes up, so does parasitic drag at the rocker.
When buying your new manifold buy the Vortec version to match the raised thinner ports and the ten bolt intake pattern with a different angle than the older SBC heads used before 1994. !997 was the last year for the 305, and 2004 was the last year for the 350 SBC so the SBC is disappearing rapidly from junk yards now. Better buy your 350 Chevy small block while you still can.
When I first started hot rodding the SBC the flat head Ford was as common as dirt in the bone yards and people wanted me to use one of them, or the more modern Thunderbird Y block Ford V8 as that is what everybody else was using. Funny how common as dirt changes, into greasy dirt in junk yards, that have product for sale only so long as they have customers buying the parts. Everyone today is buying an LS-3 or an LS-7 gen V small block Chevy, not 350's out of antiques. So I suspect that they will be gone soon enough, making a SBC as hard to find as a Flat Head Ford engine is today.