They're supposed to be able to be driven in with a hammer and a drift punch. If it's hard to move, a little heat and a bit of penetrating oil might be a good idea. If the inside of the crankcase is flat enough, a C clamp and a socket the right size to push the plug in might be a less dramatic method of getting it to move. Once it's pushed in, getting it back out for some reason might be a bit of a challenge. One of my 261's has the bypass plug, and I'm considering trying to drill and tap the plug so it can be moved out as well as in. I'm also considering threading the hole in the block and making a screw-in bypass plug from a socket head Allen Bolt. That one will have both full flow and bypass oil filtering.
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!