To add on to what
rossvtaylor said about autochains, they are only good for maintained, paved roadways with less than 4-6 inches of snow, which is why they are frequently found on school buses and ambulances. Deeper snow will interupt the spinning motion of the autochains and cause them to not deploy under the tire correctly. The same goes for not-flat roadways. They are also REALLY heavy. I'd say the set weighs 200 lbs or so.
My ambulance came with a set of rotochains. I never used them and needed the weight savings so I deleted them. I don't miss the chains continuously clinking away either.
Here's a good video:
https://www.youtube.com/watch?v=IPd3O5_SJEs
I have a walkthrough and while I use it quite often to move in and out of the cabin, I could see myself not missing it if I didn't have it. I think the dogs like it best as they like to sleep on the front seats at night and enjoy being able to see out of the front windows while we're inside the rig. That said, I "use" the walkthrough the most not by walking through it, but rather by looking through it in my rear view while driving. While I'm comfortable driving a vehicle without a rear sight line, I prefer having as much view of my surroundings as possible when driving.
I'm sure the seasoned veteran ambulance guys on here can provide better and more accurate information, but I'd imagine the only hangup for cutting a walkthrough into the cab and box would depend on how far apart the studs at the front of box are spaced, and how the box is attached to the cab. With that said, is your cab attached to your box or is separate like a pickup cab is separate from the bed? If they are separate the box will move independantly from the cab and keeping it water proof will be impossible. If the box and cab are fused together waterproofing shouldn't be an issue.