New shower 1 1/2 inch narrow. HELP!

I am replacing an existing shower with a National fiberglass 3 piece unit. The old rough in opening is 1 1/2 inches wider than required by the new one. I must split the difference on both sides to keep the drain centered. I must shim 3/4 inch on both sides. Can I use 3/4 exterior plywood to do this? I plan to totally cover the stud walls with plywood. The shower will be trimmed with cement board and ceramic tile. Am I going to create a moisture problem by doing this? I dont want the plywood to delaminate.

Thanks
Chuck

Rip 1.5 inch strips of ply and just cover the stud faces. Saves some money, and a tree. Make sure it's exterior glue.