I read somewhere on TGO that certain TPI cars (maybe certain year vettes?) got water resistant OBD-I PCMs in the engine bay, but I've never seen one.
Now you have.

I try to mount the ecm to the cab rather than fenders but they tend to be pretty durable. And IME if they exhibit problems related to vibration it's due to a problem which was already present. But make sure the wiring has some room for expansion and flexing. Leave a drop or sag in the harness where it passes from fender to body or fender to engine. Vibration will do more harm to a harness than an ecm.