A 250 block is a lot shorter- - - -that's why the 292 has taller side covers. Why not use flanged sleeves that project a few thousandths above the top of the block and seat directly onto the aluminum spacer plate, like Volkswagen air cooled engines do with the cylinder head to barrel junction? LA Sleeve can make just about anything you can dream up. You can use a thin solid copper shim gasket if necessary to seal coolant. I'm working on adapting a Chrysler PT Cruiser twin overhead cam cylinder head to a Ford Model A engine block, using a similar adapter plate and flanged sleeve setup.
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!