So...I rigged up a pre-oiler using a garden sprayer. Pumps in about 25 psi into the oil gallery fight at the oil pump.
Worked a charm and oil eventually came out of 14 out of 16 of the pushrods in the head. One pushrod in each bank did not show any sign of oil.
I did rotate the crank along the way.
Ran maybe 10 quarts of oil to prelube.
Do I need more pressure or should I look for a problem?
Once it's running, does oil come out of the remaining two?
As long as you got some oil in there you are good.
You're definitely in the minority pre-lubing an LS. Most people just start em up.
I built a SBC oil pump in a bucket pre-luber for my recent start up; ran it until I had oil pressure and maybe a minute past that. Didn't verify oil at the pushrods since it would've been a pain in the ass with my setup. Engine started with instant oil pressure and no tappy tap noises.
The most I ever really do is pack the pump with petroleum jelly or assembly lube and prefill the oil filter, and crank the engine until I see pressure on a mechanical gauge. A lot of people don't do even that. Really as long as there is some layer of oil or grease on all of the bearings, that will suffice. Oil pressure is mostly to flow oil through the bearings for cooling purposes, the hydrodynamic wedge keeping the metal apart happens no matter what. (How many engines had splash lube for everything? IIRC all Hudsons were splash lube and they raced them that way)
That said the nice thing about prelubing is being able to find out if the guy who assembled the engine forgot any oil passage plugs 
Not really a problem on an LS, but I had a customer supplied 327 that I ended up pumping six quarts of oil onto the new clutch before I realized there was a problem. Still not sure how the oil got around the flywheel, but the disk was soaked. D'oh.
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