Well, if you are ever in an accident, that involves speeds greater than 5MPH, chances are, your bumper (stock or fg), fenders and hood all have a chance of getting damaged. Same goes for the rear parts.
If you want the air shock protection of the stock bumper, you can get one of our potential suppliers to fab you a setup for the rear using the pneumatic shocks from the front of your car. If you hit some with your front, well, then you are at fault...so it will seem to most officers that shows up at the scene.
Fact is, if you want protection you can build one into the conversion. You don't need an engineering degree to build it, just some determination. But if you need an engineer, my rate is $125/hr plus travel and lodging.
As for the slots in the rear, I'll ask.