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.
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.