The Fiat Ducato produced in Europe has three roof heights; low, medium, and high. Ram sells the low and medium height versions. Ideally, the high roof version that offers a maximum internal height of 7' 1.5 " (85.5") would be better suited for a van conversion. Installation of a subfloor in the rear of the van, 8 inches high, provides additional space for tanks or storage while still maintain the 6 foot plus head room.
This second link provides Fiat Ducato specification including Fiat's mpg ratings (note these are imperial gallons, divide by 1.2 to convert to US mpg).
As far as I know Fiat Ducatos have been manufactured in Mexico for some time, but years ago when I looked at their specs it did not include the European highest roof. I'm guessing the factory wasn't set up for it, and now neither is the ProMaster's.
I agree the H3 roof would be a great option. It would allow larger tall cabinets, larger windows at top to see out while standing, and enough headroom to install a permanent overhead bunk with enough room to sit below it.