1. Measure the diameter of the hole in the block with the step in it for the plug to seat against.
2. Cut a disc from 3/32" thick brass sheet slightly larger than the OD.
3. Turn a template on the lathe with a slight dome in it the size of the stepped hole.
4. Find a big 3/4" drive socket slightly larger than the disc, and use a hammer or a hydraulic press to put the dome into the blank plug- - - -about 1/4" deflection is plenty.
5. Use a disc sander to adjust the OD of the plug until it's a close slip fit into the hole.
6. Use a piece of flat steel plate and a BFH to expand the plug into the hole- - - -use a little Permatex Super 300 to assure a coolant-tight seal between the plug and the block.

The next time the plug needs to be changed, your grandkids can deal with it!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!