Hello & welcome! I don't know all the ins and outs, but I think as long as the Lacrosse has the 3800 motor it should more or less be a direct swap over. I personally wouldn't bother swapping powertrains just because one is lower miles unless there is something wrong with the Grand Prix's engine or your doing a lot of modifications to the one from the Buick before swapping in. These 3800's with proper care are known to rack on the miles