Been there, done that on more questions than answers!
For myself seeing how others have tackled certain things either led me directly to a solution or gave me ideas how to do it suitably for my van. Do keep in mind I don't have a Sportsmoble-like conversion, just a work-only cargo van but many things here have been extremely helpful none the less. For what we're doing this is one of the best sites I could imagine. Kudos to all who've helped in one way or another. So then........
If there's a "disadvantage" to the direct-on-body window install then building the spacer ring followed by the wall covering it would be IF the window needs replaced its a lot of work undoing all your improvements for that relatively small operation. Other than that consideration there's no real huge advantage one way or the other.
Another thing to consider is testing for water leaks through the outside window flange but two things can help reduce that situation. Instead of using the butyl tape use a high grade silicone sealant/adhesive between the window flange and body sheet metal; it doesn't degrade over time like butyl can/does.
Once the window is installed but before anything else is done inside the van water test the bejesus out of it. This does NOT mean with a high pressure sprayer, simply a garden hose running on the installed window for quite a long while. (Just a standard hose nozzle squiring a stream with no more than city water pressure is fine.) Once this test is done leave the van sit over night and do it all once again the next day. Assuming you're leak free (and using silicone sealer/adhesive) you should be good for the life of your van and build.
So YES I am saying using the typical 1/8" clamp ring will be okay.
I will PM my phone number in case you need a quicker answer to something.