I have a slightly different method for closing the extra pushrod holes. I use a 3/4 in wood dowel sanded down to 18mm to plug the lifter holes. I drill a 3/16 in hole through the dowel and use a long 3/16 bolt. I have fender washers on both ends of the bolt and a rubber fender washer on the outside. Then I use an original pushrod tube seal under the rubber fender washer. I can put some sealer on the outside seal, tighten down the bolt with a nylock nut and it should never leak.
Norm Heistand my .02