Just in case anyone else ever thinks of doing this... don't, unless you have some patience & don't mind something which will turn out
not to be the plug-and-play job you thought it was.
What follows intentionally does not cover anything about the electronics involved. The speaker, amp, and remaining electronics I used in this case are another ball of wax entirely, which I may or may not get in to at some point. Anywho...
So... you have removed the dash & everything that's in the way, only to find out that there's no way in hell the sub box will fit without modification thanks to the floor vents which only the 84s possess.
Disconnect the cable which operates the vent louver, and remove the shroud over the vent. You'll need them later, but for the moment they're just going to get in the way.
The sub box comes with two brackets. Drill out the rivets holding the the bracket which attaches to the lower dash mount, move it up by approximately 4.4cm, and attach with new rivets. One corner of the bracket will need to be bent slightly to clear a bump in the box's shape.