'Vortec' pattern has to do with the intake bolt pattern, not the valve covers.
Old school heads (<1986) had 12 bolts, two at each corner and two in the middle. The middle ones were a pain to install because of the bolt angle and the carb pad sides left not much room to get a wrench or socket in there. I've used header bolts with the smaller bolt head with success.
From 1986 to 1995 GM played a nasty trick on us. On the TBI/TPI intakes, GM retained the old 12-bolt intake manifold bolt pattern, but changed the angle on the inner four bolts to make them easier to install. Find a nice late-model truck 350, and your cheap Edelbrock Performer intake you bought at the swap meet does not bolt up right without some ugly die grinder work. (Upper left corner of photo - the two dark marks above them are the Vortec manifold bolt holes - had to be plugged with set screws.) These heads weren't so good in the flow department, so it was better to find some old 'camel hump' heads (two bumps cast into the end of the head) or 'fuelie' heads (because they were common on the mechanical fuel injected engines) from the late 60's. Now a pair of these that are not modded or used up are hard to find.
Vortec heads have eight bolts, two at each extreme corner, none in the middle. Vortecs are reputed to flow as well the old fuelie heads that it justifies buying the intake. Some people redrilled the heads to use the old school intake manifold pattern, but coolant leaks were common with this adaptation. Vortec-pattern manifolds are so common now that cost is no longer a driver.
One more quirk that applies to your question: For some reason, GM continued to use the old school intake manifold pattern (not angled) on aluminum heads such as the Corvette and Fast Burn heads. I think these are what are used on the ZZ motors, so you can use that swap meet Edelbrock manifold.
Before 1986, the valve covers had the old school four perimeter bolts. These required the speader bars to even out the torque on the cover rails, or you'd get a leak, primarily in the back corners because the engine tilts back a few degrees towards the firewall.
Does one need 2.02/1.60 valves? Not if you have a 305 - runs into the cylinder walls. Also, smaller valves have a higher velocity than the big valves, which helps with low and mid-range torque. Unless you are running at high RPM or have a 400 SBC, you probably won't realize much gain from the expense.
The center bolt valve covers introduced in 1987 had and uninterrupted silicone rubber gasket, and the four bolts down the centerline of the covers could leverage the entire tin valve cover as a 'spring', keeping tension on the sealing edge. They seal a lot better. GM ironically used the same tin centerbolt valve covers on Vortec heads.
Some aftermarket heads have both patterns drilled. My S/R Torquers aren't compatible with the factory tin centerbolt covers because the lip hits the perimeter bolt pedestals, but I could use cast aftermarket ones. I'd have machined them down but I did not notice until after I put the engine together, so I used perimeter tin until my pockets get deeper.
The one trick I'll point out with GM tin covers is that you must use the narrow body design if you go to roller rockers. Tall aftermarket centerbolts are available, but the same problems applies.
