I would take a different approach.
Fitting the chair to the hack will be simple, the hard points you've got to bolt to are known. Fitting the chair to Rick will be somewhat more difficult, he'll have to be comfortable and the restraint systems will need to be fitted to him. Building the chair in Washington and riding it down to him might work out fine, but that depends on a certain amount of luck, and as my grandfather used to say, "Luck is a crutch for the unprepared". I would find someone who can fabricate the chair in Rick's area, he'd be able to test it for fit and function before you arrive. In the meantime, you take some detailed measurements of the hard points in your hack so the fabricators can build a drop-in frame. Perhaps a bit easier, you have a mount built local to you and fit that to the hack before you leave and provide dimensions to the folks in Texas so they can build the chair to bolt directly to that.
Its a little more complicated this way, but it is more likely to work the first time you try it out.