Get a Ford 6G large case alternator rated for 140A. I believe if you search for a F450 6.0L it will come up with what you want. You'll need to relieve the center bottom of the alternator bracket to eliminate the interference with the larger case, but after shaving off less than 1/8", it'll clear just fine. As far as manufacturer, Motorcraft rebuilt isn't a bad way to go. Or you might be able to find an ambulance take-off that would fit the bill.