When I mentioned the shaft mounting hardware and cylinder head I was thinking of the rear bolt, but actually those are present on the hydraulic head as a poke a yoke to make sure the rocker arm shaft is set in place correctly. I thought that necked down rear hold down bolt allowed for more oil flow. I know that when I went the other way, made a hydraulic head into a mechanical lifter head I had to put a restriction in that passage to reduce the oil flow to the rockers, but maybe the increased oil flow is due to the full circle groove in the rear cam journal, which the OP will also need.
