Just read through your thread and it looks like you are trying to evaluate ALL the problems at once, DON?T do it.
Clear you mind of all the problems until you have a clear view of what is really needed, First job, get that body OFF, there is so much to do to the chassis that until that is a reconditioned roller there is little point in worrying about front under trays etc, Angle grinder with a 1mm disc and cut the side off or you will be there all month, it sounds brutal but fiber glass is easy to fix and you will need to get friendly with that stuff.
Once you have a clear view of the rolling chassis you will see there is a lot more than you see now.
When dropping the floors the best is the wedge design where the front is a continues slope to a 3· or 4· rear panel, if welded to the extreme width possible you will have about 17.5" for you seat, which rather limits your choice so no good looking fondly at the majority of sports seats, they will all be too wide. But that is in the distant future, basics first.
This articul has been read a few hundrd times, try it.