I'd suggest get chuck senatore's old book, and ream + clean up the passages between the oil galley and the mains the way it's spelled out in there. Other books say it in one sentence, but that one explains exactly what to do.

If you've never checked out what a mess those particular oil passages are in a stock block you're in for a real treat. It would not surprise me if you found a couple holes that are 2/3 the size of the others, and that would do it.

For the type of use, I would not restrict the oil to the rocker shafts with an stock (other than above) system.

Considering the preference for high viscosity oil/additive, that stp adds a whole bunch of viscosity....but if you like it, go ahead and use it....I also think it's a good candiate for the old direct connection "hemi" 1/2" oil pickup, radius and blend the pickup passage modification. SAE 50 and 40 weight were commonly used back when the books detailing those mods came out.

All that will give you more flow (volume, not pressure) through the system......it should definitely help