Hello Pete,
The way software programs like Flexi works, it is not a simple drag and drop of an output driver into the program especially if the desired driver is located in a newer version of the software package. Too many factors, like engineering changes, play a major factor in making them incompatible. I recommend that you determine which software your printer was released, and install that version in a fresh directory instead of trying to patch your copy.
In my own research of your issue... I reviewed the Readme for Flexi 8 (I no longer have Flexi 7's), I noticed that in the earliest version, 8.0v1, SA International released the JV3 SPII drivers for the Mimaki. This leads me to believe the JV3-160SP may be developed in at least the last version of Flexi 7... 7.6v2. If you can obtain a copy of this CD, you will be able to use your Flexi 7's user # and password to install on your system and drive your printer... without having to purchase an upgrade.