To the best of my knowledge (which isn't terribly great) the only O.E.M. "street" (not "marine" or "industrial", etc.) 427 cu. in. hydraulic cam engine examples were in the 1968 Mercury Cougar 7.0 Litre GT-E's.  But that was a "side-oiler".   

       Scott.

A PVC pulls a vacuum if functioning correctly - running with open breathers will INCREASE crankcase pressure as the vapor has to be "pushed" out of the breathers vs the PCV drawing it out.

What do you mean rubber seal, there are several types of main seal materials? Also what type of side seals are you using? The elastomer side seals don't work very well in my experience and I've had much better luck just using RTV to seal them.

I doubt you will find any seal material that will fix a rear main leaks that happens within 50 miles, that would indicate a mechanic issue. Bearing bore, crank finish, crank seal diameter, etc is an issue and on a seal problem. You can still get 1 piece rear main seals for an FE I think, but I've never really found it's any better than a good quality standard seal with RTV to seal the cap.

I agree about the thicker wall.  And the sonic check before boring.

I thought and 1/8th inch wall would be more than enough.  Probably use a 4.13 bore so it could be overbored later.

I have never seen a center/top oiler with hydraulics, at least one that wasn't drilled by someone later

Side oilers I have seen, 68-72, not sure if they continued later, but D2 has been the latest I have had my hands on.

This issue has been showing up more and more.  I think it is because of pictures attached to the post that are too large for the software to handle.  But not sure about that.  I have seen the same thing, where when logged in you can't see the post, but if logged out it shows up.  Weird, and unfortunately I have no solution...