When done properly, a restrictor much smaller than 1/8" will be needed. Somewhere in the .040-.060" range is where it should be. You'll have to start small, and work your way up.
I am curious.... what would be the danger of starting larger and working down in restrictor size? Overpumping oil to the heads? I would be concerned at starting too small and perhaps burning something up....
BTW, agreed on the size above. The stock flow restriction mechanisms (cam 'interruptor' and flow area restriction into the rocker shaft) ROUGHLY equates to the flow in a hole in a .05" or .06" hole with constant pressure. But that is only a rough number as there are a lot flow dynamics involved and so that is only a starting point. I'd venture to start at maybe .060" or .080" if I wanted to have more than stock flow.