Gremlin/Hornet floor pans in Spirit?

I don't have any experience with replacements, but gee, they should be the same or very close. An Eagle will have a different floor pan to make room for the transfer case.

Not sure if the catalytic converter on a Spirit raised the floor on that side or not?

Ok, to partially answer your questions. I have worked in the auto collision and restoration business for a long time. With that said there are many variables in restoration work. Your skill set, tools and of course your budget is the big one. Also the level of restoration that you want to accomplish. If you want to exactly replicate the factory contours, seams, etc.. etc..  then be prepared for a much larger budget. If the objective is usability that looks good, then you have a lot of options. Using a floor pan that is similar would be the alternative. 

Next you would ask yourself if you want to replicate factory seams. This might require more work in some cases. Example: Many AMC's have a seam on the front floor pans that runs side to side on the car. The location is directly under the heal of your shoe when driving. It is the place where the floor pans meets the upward angled floor pan/firewall. Some other cars such as a early mustang do not have this seam.  If you replace the entire floor pan in your car and the rust is higher then the seam ( always is), you would need two patch panels. If the replication of the seam is not that important, you could use one patch panel.

A few things to consider and to do; 

measure the floor pan, width and length

note any contours, body lines, offsets, holes etc...

note factory seams - where on panel ends and the next begins- especially the amount of overlap

are the adjoining panels in good shape or rusted as well

Once you have some idea of the shape and size you can research similar panels to use as an alternative. ( I personally like the early mustang floor pans as they are close in general shape to many other cars including my Javelin and very affordable)(did I mention available and affordable)

Another item to consider, regardless of what floor pan you get (even a Spirit panel) you will need to trim, finesse and make alterations to the panel before you install that panel. The internet panels for sale leave a lot to be desired and require extensive amount of finesse to install ( ask anyone who has installed the trunk floor panels for a Javelin)

Regardless of the floor pan you use, everyone of them will require work. Cutting a few relief lines, adding some extra metal, etc is all part of the game