Recommend marine quality stainless steel fasteners and backing plates. The latch can be mounted like the photo above (as mine are) or upside-down depending on other brackets on the door, wall, etc. The existing mount holes for your screw type can be reused at least partially.
As for mounting them at different heights? Sure. They'll look dorky but you can do this any way you want, even double them up.
The issue I have is the latch will not fit in the gap. If I were to shim it from behind I don't believe the latch would apply enough pressure to seal. Do I open open up the gap to allow it to seat flush on the body? Any one have the same problem? I hesitate opening the gap, suggestions?