Use a flat-head Allen bolt with a tapered head in the crankshaft and taper the hole in the washer. It will eliminate the bolt head sticking out. A better fix is to use Pre-68 Dave's water pump relocation plate that adapts the 216-style water pump to the later engine, raises the fan about 3 inches, and also centers the fan on the radiator for proper cooling. Electric fans are a solution to a nonexistent problem when the right fix is simple, easy, and much more effective.
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!