The pan has already been sliced:
The cut is nice and flat. I'm going to go pick it up next week, at this point I'm planning on just CADing up the plate in Solidworks which I decided to teach myself to use last week. If I CAD up the contours for the new bottom I can replicate the ribs of the Fiat pan which I think will be pretty cool, not that anyone will ever see it but I'll know. Oh and the dirt skirt ribs are gone.
The pan rim is about 3/16 give or take and I plan on doing the new bottom, probably our of 3/8ths with the ribs taking up about half the thickness. I plan to leave some extra girth where the Fiat pan mounts. A bolt on bottom probably wouldn't work too well, I don't think there is enough sealing area given the placement of the rail bolts which would probably bow out the pan bottom, You'd have to completly goop the thing up with sealant. To really make a wise joint like that you need close tolorances, not a lot of flex and something like Hondabond which is specifically make for mating machined surfaces without physical gaskets.
I thought about doing a full milled pan, but we're talking a really hunky chunk of aluminum even if you are just doing the top pan section without the sump, you're talking maybe about a grand just for the block of 6061 aluminum.