you can get a 5/8"-18 oil drain plug and a copper washer. ran mine like that for a couple of years. Or if you make the end of the bolt pretty much perfectly square with the axis of the bolt, (I.E. turn it on a lathe or very, very carefully on a grinder) then put a copper washer underneath between the bolt and the shoulder in the block. I wouldn't just put in a bolt as the ends of bolts aren't really too well finished. Either way would work well. I am pretty sure I wouldn't trust permatex in that situation, it was never intended to perform that kind of service.
