Hi captinjohn,
I just bored out a couple to 5/8" the other day. First beveled the edge with a swiss file. Then I put it in the vise and used thr drill press to drill it out to 9/16". Beveled the edge again and drilled it to 5/8" with no problem. Clean the bore up with some 240 wet or dry, then dressed the gasket surfaces with sandpaper on a large piece glass. If you do the trister heatdam (part number PS04357) you will have to fill the open wells with JB Weld first. When done the JB will be exposed in places in the bore. Works great.

Dave