Don't forget that a Spirit body is made with a permanently bonded-in female insert that the upholstery insert then fits into.
The bonded-in insert is what gives the body so much strength and rigidity without adding much weight.
It's only the upholstery form that fits into that bonded-in body insert that can be put in and pulled out.
If you're looking at just the upholstery form to use a a seat, it isn't strong enough by itself, it needs the body insert bonded to the body shell to give it the needed support.
I have a Spirit '27, which is made the same way.
Since you have a wide body, if you really want to try the Spirit insert, you could ask Josh about the '27 insert which may be wider than the '23 insert.
Here's some old videos from Spirit that really show the bonded-in body insert and the upholstery insert and how they all work together, they may be helpful as you make your plans:
You may need to add at least part of the body insert, rear half or so, into the body to be able to use the upholstery insert . . . Just call Spirit and ask Josh.