Just food for thought here. My son rebuilt a magnum 360. Ball honed , and a standard forged piston. Im pretty sure 1/16 rings. Your standard moly type off the shelf piston/ring.
He put an evac system on it, and a vac gauge on the valve cover. Of course it produced little to no vacum. I told him get rid of all the rubber gromets etc. He fabbed some alum pucks with the centers tapped for 3/8 pipe. He welded these onto the valve covers. Got some -10 AN fittings and hose and plumbed it to the headers. Then he put one with a check valve to a breather tank so if there was positive pressure it could vent. This AN setup produces some vacum ! At idle the gauge just comes off "0" . At 55mph on the freeway it has 3" of vac. At the track depending on the air it produces 3-4" of vacum at the stripe. I was impressed. The key is sealing up the connections at the valve covers , as much as you can