I wouldn't mill it much at all, not really that much point in going to higher CR's.

I would have them put in hardened exhaust seats though. And maybe a little light exhaust port work? Many B20 heads seem to have an intentional choke in the exhaust port. Not sure of the purpose of that, but it's pretty obvious and easy to remove. Intakes really don't need much work at all.

While the motor is apart, I would get a very accurate measurement of the piston deck height (measurement of the piston at TDC either above or below the block surface) and use that as a guide to selecting the proper headgasket thickness.

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'63 PV544 rat rod, '93 Classic #1141 245 +t