I am converting my Spirit to an SX4. The front frame has holes drilled and tapped for both the Eagle and Spirit crossmembers. The only difference is the floor pan. I absolutely love the Spirit GT armrest and gauge cluster, so it is worthwhile to convert a Spirit. The gauge cluster cannot fit an Eagle because the Eagle floor pan has a small hump in it in front of the shifter. That brings the whole ridge up a half inch. That space is needed for an automatic shift linkage, but I won't need it because it will be a manual transmission. A Lokar shifter would also have the clearence if I wanted to make it an Automatic. When I do the conversion I am retaining the center of the floor pan and cutting out on either side of the hump. Once the transfer case is in place I will fabricate a new hump around it. I will install the drivetrain before fabricating anything since the engine is also offset to the passenger side on Eagles, but centered on a Spirit.
You could splice in a Eagle floor pan. It is not as easy as that sounds. The Eagle has support rails that connect to the rocker panels. The Spirit is missing these, and it will complicate the work alot. I'm doing the conversion because I love the Spirit armrest, so thats another reason I would never switch floorpans.
The seats and seat frames are the same between Spirits and Eagles. The difference is the two floor mounts that attach to the floorpan and the bottom of the seat frame with two half inch nuts. The inner floor mount on the Eagle is shorter because it sits on the hump.
I want to eventually buy a 1979 AMX and bring over the armrest, dash overlays, and front AMX grille. If I get good enough at fabricating I will also mimic the AMX fender flares on my "Eagle AMX."
-Dave Spillane-