Reworking the 273 Adjustable Rockers

RR, if the holes are on the valve side, you may have the shafts reversed side-to-side.....???

Regardless, IMHO, a hole in the .090"-.100" range is adequate, and a .12" should work OK too. Keep in mind that with the stock oiling system that there are 3 places where the oil flow TO the rocker shafts is limited:
1. The 'oiling interrupter' holes in the cam: .049 sq inch flow area and about a 5-6% duty cycle per calculations and some estimating
2. The head bolt to oil passage clearance inside the head, approx .050-.060" sq inch flow area with standard head bolts; this will get restricted down further if the ARP head studs are used, since the studs are considerably thicker than the bolts.
3. The clearance between the rocker shaft hold down bolt and the hole in the rocker shaft through which it passes; about .034 sq inch with the stock 273 shafts and 5/16" hold down bolts.

So the rockers will rise in level to some level due to this pulsating oil flow and the level will fall due to the outflow from the shaft. With some RPM's I think the shaft will fill fairly well, BUT for the pushrod oiling holes to be effective, the shaft has to fill higher since it is 'uphill'. So some checks on the restrictions above are in order too.... which I think is one of Yellow Rose's points in all of this.