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Look at your propellers the will give you out put rotation

The ZFs (at least the ZF IRM 220As) will run in either direction, no difference in ratio, so it really does not matter. In twin installs one runs in reverse to acheive counter rotation.

Someone asked this years ago and I looked at mine to see the position of the shifter lever on top of the gear and remember that they both pointed to the outboard side when in forward. Pretty sure the port one runs in reverse with Cummins 6BTAs.

UV

Depending on your controls, it can be as easy as switching the orientation at the helm controls.
If they are wrong it should not take you anymore than 15 minutes to switch them. As others have stated the ZF transmissions turn both ways.

The ZF trans do not have a forward or a reverse, they just spin CW or CCW and running them either way does not hurt them in any way. They were specifically designed that way.