If you're going to drill a drain hole you might as well take the extra time and tap it for a drain plug. Grease the drill and tap to help prevent shavings from entering. The early converters were tapped 1/8" NPT but they changed to 1/16" NPT in the mid seventies.

I've seen converters cut open after flushing with commercial flushing machines with the air blast feature and you'd be amazed at how much gunk is left (some of it loose).

The only to get it really clean is to cut it open.