Since the later-model high-pressure 235's aren't that scarce or expensive, it makes better sense to upgrade than to undertake an expensive and somewhat risky modification of the older engine. The only advantage I could see to the conversion would be cosmetic, since the appearance of the late 235 is a dead giveaway that the engine has been changed. The tall pushrod cover can't be fitted to the late model engine to make it look like a 216/early 235.
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!