Where did you place your engine and are you using the original driveshaft? I wouldn't be placing any cab on the S-10 mounts. Those get cut off and you can build new mounts wherever you need to put the cab.
All the S-10 swaps we've done involved putting the motor where it needed to be in relation to where the cab needed to sit. None of them sat in the original position, but all used fenders. These aren't the prettiest front end to go fenderless with, which is why most don't do S-10 frames with fenderless vehicles, but that's a personal choice.
I have no idea how much room you will have for the cab once you shorten the frame/box, you'll need to do some measuring and figure that out. You'll definitely need to move the back cab mounts/front box mounts, and I would plan on moving the front cab mounts to get the cab exactly where you want it.
You should have a good look at your frame and see where the factory section is that slides together. It's not cutting the "box", it's cutting the part that slides together to cut off what you need and put the frame back together. It's easy on the S-10's if you do it in the right spot. It's how they so easily had 5 wheelbases. You'll probably need to make custom mounts for the cab wherever you put it.
Do a lot of thinking, measuring, plotting, and figuring before you pick up any tools, so that you're sure what you need to do.