Go to or email a reputable machine shop who builds oval track dirt or pavement Chevrolet Vortec Racing engines.
Let them source your heads as bare Vortec heads. The ones they recommend not the ones you think will work.
They will have a list of duds to avoid. They will build you affordable heads from known good castings which can save you money because they buy in volume and do mainly Chevrolet Race Car Dirt Track Engines.
You'll be supporting the good guys who run the racing industry too. Best option for higher quality at any price.
Lil' homework and reasearch required. Call them yourself. Tell them your predicament. Do not make up a story.
If they are less than $1200.00 plus core, for a discount pair, (plus delivery), you are buying from the wrong machinist, in my humble opinion. You might be able to negotiate the core fee or find some other heads he will accept as cores. If he is out of town pay the core charge or try to locate a trustworthy local machinist and meet him in person.
If you want to ask him to select some extra quality castings and perform some of his race head magic, you could expect to see $1600-$1800 a pair. It will be worth every penney.
Keep in mind, if your local shop sources the same crap castings in you post, you'll end up with the same crap you are trying to avoid in this thread. Make him prove his source and ask him for time to verify his supplier's reputation and then get everything he says in writing. If he refuses find another machinist.
Pay the nice man for $120 stainless valves, proper valve seals and quality springs and retainers, or you'll get ripped off with substandard valves and springs.
Do not insult him with worthless FleaBate cores, or you'll make him angry. If you buy from a circle track machinist, you might be better off just paying the core fee because shipping cores will not be pretty.
Chevrolet Performance Fast Burn Cylinder Heads 19417592 run $1293.00 EACH for a reason.
When you race, you buy from The General and they work and they are NOT cheap.
You can certainly get a better deal than they run, but they are the real deal Vortec heads, not the fakers or duds.
The Machinist Union and the aftermarket supply system is a very powerful group of people who will teach you to buy directly from the machinist or you may learn the hard way.
You do have a choice... (Well, OK, not really...) You have no choice... Chevrolet Fast Burn, slow painful expensive burn... or pay the nice machinist man and get it all in writing.
Choose wisely.
Good Luck!