More Questions:
where are you in the country? Climate: Does it get freezing cold? if so, a DryerBox installed with no insulation may sweat inside causing water puddling.
Is the future dryer on a pedestal or on floor?
What is the exterior siding? brick, stucco, lap siding...
If you are demo'ing the wallboard, then, depending on what "mechanicals" are preventing it, I may consider adding a firring strip to each stud if it was 2x4 framing and use the 425.
Usually i prefer to help you on the phone.
Also, a great resource is this gallery of picture stories from other installs:
https://dryerbox.com/dryerbox-installation-pictures.htm Feel free to call me at work: five six one five 29-4607 Thanks