To get a more accurate idea of how much to machine the block, measure the inside dimensions of a 216 and a 235 thrust bearing. Be sure to narrow the web the same amount front and rear to maintain proper alignment of the timing gears. I have a face milling cutter setup almost completed that uses the main bearing bores as a fixture to guide the cutter. Call or email and I can send some pictures of the setup I'm designing.
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!