It is doubtful that the inspector will approve the space. Per NEC 110.26(E), the minimum headroom is 6 1/2 feet. An exception there might cover your situation ( my interpretation is that the exception does cover you ). But most of the inspectors I encounter use every trick in their book to deny that the exception applies to the situation at hand. Better clear it with your inspector first. I have even "shopped" for inspectors as lawyers "shop" for judges.

I'm not sure a flanged inlet device would qualify as equipment needing the 110.26 clearances but you can try the inspector.

NM can be used as long as it's follows the surface and is not subject to physical damage.

There's no requirement for plywood behind the box, follow the instructions however. Sometimes it's just easier to do it that way.

"There's no requirement for plywood behind the box, follow the instructions however."

It does make attaching any required clamps a lot easier.

It does make attaching any required clamps a lot easier.
If you can shoot the pins to attach the box to the wall, you should have no problem putting the clamps in either.

Thanks for the advice. I'm waiting for the inspector to return my call, then I'll post an update on what he says. If anyone has any other thoughts in the meantime, have at it. Thank you.

"If you can shoot the pins to attach the box to the wall, you should have no problem putting the clamps in either. "

It is a lot easier to shoot a few non-critical pins to fasten up some lumber (I actually prefer 2x) than to fasten clamps.

I have fastened all sorts of things using pins, anchors, and tapcons in commercial work.

It is just as fast in residential work to shoot a few pins to hold up wood, then stop making loud noises in the customers house and go back to a drill-driver and set screws (and not drywall screws).

Why do you want your inlet to be inside though? I'm assuming this is for a generator, which is run outside.

It's more convenient to mount it inside the crawl space because that way I'm mounting it directly to the outside of the basement wall. Less 10/3 cable to run, easier to mount without disturbing the siding, protection from the elements, etc.

I presume that the crawl is open to the outside then so the generator cord can get through?

Yes. There are two barn door-style doors that lead to the space. In fact, one of the doors is warped slightly at the bottom which provides the perfect space to lay the generator cord.