All 4-door models (sedans and wagons) appear to share the same floorpan – from edge of firewall to trunk (* see below). 2-door F and M bodies also appear to share the same floorpan - which is shorter than 4-doors. I “suspect" a person can modify a 4-door pan to fit a 2-door without too much work.
J-body floor pans “should” be the same as F and M body’s – but, honestly, I’ve not done a comparison to make sure. I do know that some J-body transmission crossmembers are (completely) different.
(*) There are two different design trunk pans - those with round spare tire hole ’76 to early ’77) and all others (late ’77 to ’89). The piece that connects the floor pan to trunk pan “I believe” is the same for all FMJ-body’s.
From my early '77 wagon.
Late '77 to '89 version (also a wagon).
I’ve known a few people get newer (mid ‘80’s – which would be 4-door M’s) floor pan sections, and weld into their F-bodies without fabrication (other than the usual cutting and welding).
M-body (sheet metal) doesn’t come from the factory with the factory installed “rust option” (on most models) – like the F-bodies did.
BudW