I've removed them in the past by driving them in with a punch then levering them out with a pair of channel locks (aka waterpump plyers). Hit one side down causing the opposite side to tilt up so you can grab it with the plyers. Now keep in mind this was in the water jackets of a V8 with it tilted up on it's side, so they didn't have far to fall (if they did fall in) since the block (cylinders) were only an inch or two below.
That proceedure may be a bit risky in your case since it could fall into the crankcase. (I'm kinda new to these old sixes so I'm playing it safe and assuming that) I would suggest using a small [censored] punch to make a hole. Then screw in a sheetmetal screw (lag screw) into the hole, then use a long handle screwdriver, (or visegrips, or channel locks) to pry the freeze plug out. If you notice the freeze plug sinking in, stop an drill a hole for the sheetmetal screw / lag bolt. Use some thick wheel bearing grease on the bit to prevent any metal shavings from falling into the crankcase.
