Well it's a truck and you're converting it, right
![Big Grin](https://www.truckconversion.net/forums/images/smilies/biggrin.gif)
I've heard of people getting an "accordion boot". I believe that there are two thicknesses, the bigger for when the cab is air ride, the smaller for when the cab is fixed.
That's the end of my knowledge but it should give you some search terms.
Sounds like a cool project, don't forget to share some pictures.
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