I use a piece of 3/4" thick steel plate bolted to the back of the block to mount a Van Norman cylinder boring bar for cutting the counterbore in the back of the block and the main cap. I've modified several blocks for the one piece lip seal that way. The boring bar cats' paws center up in the rear main bearing bore, and I use a square tipped sleeve cutoff tool to machine the counterbore to the right diameter, 3/8" deep. Specifications on the lip seal claim that it can handle up to a .015" +/- diameter shaft.
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!