Front end similarities hornet/concorde/eagle?

You'll notice in that pic, that I even left a couple of tabs on my cut that kept the original seat mounting holes....

It should - because as I pointed out - the seats have the same mounting points save for the front center on each seat. To do that the floor pans would have to be at least very similar if not the same in the other three mounting points for each seat, same relative horizontal plane as well as vertical plane otherwise the other three mounting points would have to be different. Since the only difference is the height of the brackets (slight difference in height only) and the front tunnel mount, that tells us the floors of the Spirit and Eagle are pretty similar - the same in most spots. 
But again, we're talking Spirit and Eagle........ 
Dash is the same - showing the firewalls at or near the top are the same (minor differences for clutch cylinder mounting point), sill plates interchange across two door models, hoods, hatch, side glass..... same. 
the two door sedan seems to be different from the hatchback models - carpet for a sedan doesn't fit a hatchback for !##!. Not even close behind the middle of the front seats. (carpet fitting in an Eagle is a pain to begin with but add to that the limited options and it's no fun at all)

I've pointed out the seat frame - mounting rails differences, they are different as can be seen in the photos. Spirit vs. Eagle - lower frame is quite different in shape, etc.. 

Look at the photos again and tell me the Eagle seat outer mounting provision isn't taller - and the frame height different, the shape of the frame different. Yeah, they DID make different seats. Can you make them work - yeah. But they aren't the same. The outer mount of the Eagle had to change to raise the seat up enough to clear the tunnel! If you put that Eagle seat in a Spirit it would sit a bit higher. 
They had to change the seat frame and make the outboard mount taller or the seat couldn't have cleared the tunnel. The seat BACKS are identical - one will transfer directly to the other (assuming same seat type) but I found the seat bottom upholstery is a bit different for Eagle, the lower edge and trim a bit different due to the height in the car. I have NOS Eagle upholstery and I have Spirit seat parts - and they don't interchange well. 



But again - this was never about Spirit or the later AMCs, it was about the earlier cars - a different car - the Gremlin.   
We were never asked about Spirit...........

PS - this needs to be said directly - MacGyver did a bang-up job on that floor and the car looks really nice from here. Pretty cool. It's a car on my bucket list to see in person, honestly. To transplant a floor piece like that is no small feat and it looks sharp. 
Much better than those who take a decent Eagle and hack it to bits to make a Spirit out of it - ugh, what a waste. If ya want a Spirit, BUY a bloody Spirit but don't hack up a rare car to make one. On the other hand - salvaging Eagle parts like this - gotta say, pretty cool and a ton of work to be sure. 
So regardless of the technical back and forth - kudos. Nicely done - and thanks for the pic of the floor - that speaks nicely. 

tomj wrote: they'll change the damndest-seeming things to avoid changing something that looks simple.

I just want to say that I fell victim to the fender differences myself too! Bought a bare NOS fender at the last Homecoming show and thought it would work on my Concord. Turned out to be an Eagle fender. So.. I have an NOS Eagle fender to trade for a bare/really nice Concord fender to anyone interested!