A 55-up engine doesn't fit an earlier engine compartment unless the water pump shaft is shortened, the radiator is moved, or a relocation plate is used to run the 216 style water pump. There will also be a fan clearance problem with some crankshaft balancers unless a special shape fan with a notch in the blades is used. If you're concerned about valve clearance, create some. A little lifter rattle during an initial engine run isn't going to make all your children be born naked, or some other life altering catastrophe!
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!