I have two of these high roof/extended rear door vans----they were purchased with those features already added by competent converters.
As CarringB says since they're ADA-compliant its 100% certain there is an upper structure welded to the E-Series body, the extended height doors will have properly installed and fitted upper latches---that's an FMVSS from 2003 onward.
I'd strongly consider possibly buying the van outright and in its present condition rather than do all this work to swap the top onto another body. There are a number of advantages to the increased headroom but your actual use would ultimately determine if such a configuration would be that much an improvement over whatever you're driving at the moment?
I do like---or think I'd like---the extended side door height but my needs have the taller rear doors being much, much more useful. In my ideal world I'd have both side and rear doors taller but to date I've never seen on like that. I gather the further reduction in upper body strength is the reason but I digress........
FWIW the extended height doors do make entry and egress much "easier" than the typical E-Series configuration we typically see.
One note about this van pictured----the side door area has been modified to be more "bus like" which I'm sure is part of the ADA adaptation package. That could be removed, the body in that area "restored" to the original factory profile.
You do want to check if the front passenger seating area has been modified from stock---that's something I see quite often in these types of converted E-Series.
Best of luck here---could be heaven, could be hell!