I'd recommend not using LSAT passages for the MCAT as they indeed test a different set of skills. The LSAT passages are much more logic-focused, and though the question stems may be similar ("the author would be likely to agree with...", etc.) the burden of proof for an LSAT answer choice is higher than that for an MCAT answer choice. Going from the LSAT to the MCAT you'd only be confused with how debatable some of the MCAT answer choices really are.
I took the LSAT in Dec 2009 and the MCAT in Jan 2010 so I speak from experience. I did extremely well on the LSAT; here's hoping at least the same percentile on the MCAT...