If one was to set up a NEW set of valve covers where and what size holes would you want installed. Keep in mind I may want to upgrade to a vacuum pump in the future.

Would you install a oil filler cap?
Would you have the breather holes installed in the front of the valve covers, and would you drill for a breather grommet or drill for a NPT fitting?
If NPT is the answer what size??

I'm interested in this topic also and trying figure out the same. I have seen that 1.22 inch hole size seems to be a common standard for breather grommets. I plan to use Moroso 68770 pcv grommets. Exactly where I'm going to drill the holes I haven't figure out yet. Don't forget to install baffles inside the VC's also.

thanks,

Steve

What type of covers, cast aluminum, steel or fabricated aluminum? Which motor?

Cast Aluminum, 426 Hemi, I am looking at the STG V or Ray Barton covers

The early (1966 to 1969) cast Stage V street hemi valve covers with out the stage V engraved into them I have now had two oil filler tubes in them stock I was going to buy another pair polished for this customer from Tim at FHO but the only ones we could get at the end of last year was for the later 1970/71 street hemi motor type which the customer didn't like so I'm having the original ones polished now
Hemiroid motors
This rascal better make 850 HP on Oregon pump swill or better to make me happy

As far as the way to add a removable oil filler cap that should seal up well enough to work with a vacuum pump or PCV take a look at Allstar Perf. oil filler cap kit Pt# 36180
It seals up with O rings on both the cap and the flange that goes inside the valve cover Summits sells them , I bought several of them for future engine builds with fabricated aluminum valve covers
IHTHs

Cab

I looked up that PN# and I liked what I seen, I found just the cap in Black PN# ALL99366. I am going to see if i can have the filler hole tapped for a 3-12 to accept this type of cap. (My other option is a remote filler )I will put the filler hole in the back of the valve cover on the drivers side, and add some evacuation fittings of some type in the front of each valve cover. I am open for suggestions there as to what size to tap the holes for the evacuation lines.

I bought a used GZ vacuum pump from Rich at Fastman EFI several years ago, I called GZ and ask about where to put the bypass valve and where to put the input line to the pump. They said to put the bypass valve right beside the pickup line so I wouldn't be sucking dirty air from the outside across the motor once the bypass valve opens
I ended up putting both fittings on the passenger side front valve cover as close as possible to the pump I did have a thin aluminum home made baffle welded under both fittings, I drilled a bunch of 1/2 inch holes in it, it runs half way down the valve cover to try and keep the pump from sucking oil into it. That seems to be working good, I get a lot, less than a lot really , maybe a ounce, of condensation out of the sump bottle when I drain it three or four times a year.
I haven't put a filler cap into it yet either
I will, soon Luckily this motor doesn't leak oil anymore or use oil, the oil stays a lot cleaner now longer also
On the filler cap I would put it on a flat surface as high as possible on the valve cover closest to the intake manifold to help the oil drain back into the motor from the top side of the heads instead of from down low, especially on a hemi motor, so you don't flood the lower drain back holes in the heads