Time for the grumpy old man glass guy to wade in here..........that'd be me in case you're wondering!
I've installed and serviced at least 50 of this type window in van bodies; some new additions, some replaced for various reasons. Take any of this FWIW...........
-Cut Out Tool:
Plenty of great ideas already here BUT for those not experienced with the pneumatic nibbling tool or 4.5" angle grinder there's nothing better than a good orbital jig saw with appropriate blades. The others mentioned require a bit of experience for good cuts without damaging the surrounding areas sometimes requiring paint touch up. The pneumatic tool and angle grinder are a bit dangerous in use especially without previous and significant experience using them, more so when things go wrong (during the cut) rather than when everything goes 100% as imaged.
The resulting opening will have rough and bare metal exposed edges so its a good idea to gently smooth them, as much for personal safety as a good practice. Once this is accomplished use some sort of autobody-type bare metal primer, one that doesn't require a cured top coat in order to remain effective against rust etc.
Depending on the nature of your build avoid bonding the glass frame to the body---it will be nearly impossible to remove it should service ever be required. By "service" I mean replacing a broken part or piece of glass--this is necessary more often than one might think. Instead use the inner ring supplied to "clamp" the window frame to the body because most DIY's don't have the tools or experience to remove a bonded part from the outside without potential and significant damage to the exterior paint. In fact considering the tools and experience I have removing the OP's window frame as installed would present a huge challenge, one I'd simply not guarantee wouldn't cause paint damage.
OP's installation is very nicely done, finished appearance top notch. As with so many of the DIY projects though thinking and planning ahead for contingencies we're sure will never befall us makes upkeep and maintenance to/on our additions. You'll thank yourself one day...........