The RHS Vortec head, which is based off the OEM casting, is better than the OEM head. I've flowed them vs OEM and if you get the pro torker model, with the multi radius seats, and 2.02-1.6 valves, it out flows the stockers by a good margin.
Also, they are twice as thick on the deck surface and have more meat around the seats, reducing the problem of the crack prone factory vortec head. If you want to run your factory manifold, it will fit, but you will need to match the manifold to the height of the runners on the Vortec heads, and do a port match job. These heads are cast in Australia, and RHS is part of the Comp Cams group, so they are good quality.
It will run better with a dedicated Vortec intake, so if I were you, I would plan on using a Vortec style manifold. There are heads floating around out there claiming to be Vortec heads when they really are not.
In order to be a vortec head, it has to be patterned after the GM design. The name sells, so be careful of what you are buying.
Otherwise, use junkyard models, but get them magnafluxed, and make sure the bone yard will take them back if they are busted. Look closely at the center bolt holes on the deck surface of the head, and look closely at the exhaust seats. I've fixed alot of these cracks, but you need the right equipment to do it.