The Leupold bases I got for the Mark 4 are 2 piece... I don't think they had a 1 piece in that line...
From what I can see the lapping is required to get spot on levelling, so I'll play it by ear and see how well the 2 bars line up when I get the bases screwed in.
The website I was going to get the rifle from has just listed it out of stock of course,so I need to do some more digging today to find a site that has it in stock and is reputable. Cheapasdirt has come up in past posts but they're listing it out of stock too...