Re: You can block off the oil port in the head, and use conventional oil-thru-pushrods to get the oil to the rockers

I'm not sure whether that's best. It saves some plumbing, no lines to fracture. However: it takes pressure from the tappet gallery, yes?
I didn't think the GMC had pressure to the tappets, only the 1954-* Chevy?
If I missed that (and there is pressure) you still have leaks and pressure loss at ever transfer: gallery to tappet, tappet to pushrod ball, pushrod ball to rocker end, rocker body to rocker shaft. I don't know how much is lost, or whether there's a front to rear bias (last tappet from the gallery feed gets lower), but any dirt it picks up will stop the pushrod oiling.
Using a single line to a distribution point allows adjustment to the volume/pressure without taking the motor down (blue rocker? more oil).