You may be right... all depends on the how well the restriction made by the cam's 'oil flow interruptor' manages flow.
I just looked at a stamped rocker shaft from a '68 273 and the hole on top and bottom are the same size. Same on the adjustable rocker shaft I have here. So that restriction would not change. (Somehow I had the idea that the top hole was smaller.....??? Gettin' old....)
