Front end similarities hornet/concorde/eagle?

No. The floor pan is TOTALLY different, the seat mounting especially on the SX4 is very different, the front cross member is VERY different. Your floor would be among the biggest obstacles. 
 
There is not room for the Eagle stuff under other AMCs. The transfer case won't fit. 
What you read is a grossly over-simplified version. And no the Concord isn't just a face-lifted hornet - there were other changes. 
The Eagle also requires a larger wheel opening in the fender - AMC had to stretch the wheel opening forward about an inch or so to compensate for the wide wheel swing-out on turns. The first year they left Concord fenders on Eagle - and ended up with thousands of rolled fender lips as the tires would swing out and fold the front fender lip inward behind the trim. So the fenders are different because the wheels make a wide swing to compensate for the CV joint near the outer end of the axle. 
Different cross members, different lower control arms, different steering knuckles, engine sits a tad right of center, different floor pan, if it's a stick you have to use hydraulic clutch as there is zero room for clutch linkage so you have a clutch master cylinder and a clutch slave cylinder. Eagles are heavy so you need better springs. I think the SX4 is about 3200 or 3300 pounds with the sedan and wagon even more. And that's with a SIX. 
You would need an entire donor Eagle - and you'd need to either use the Eagle floor or do a lot of fabricating to make things fit. One comparison is that look at the console and rally gauges for Spirit vs. Eagle - Spirit has two rows of two gauges - not enough room in Eagle due to the very high floor hump so they had to do one row with all four gauges in Eagle. 
The front differential hangs from the engine mount brackets in Eagle - there isn't a "frame" to transfer. There are three brackets - the third that bolts to the Eagle block on the left side near the back supports the pinion to keep it level, the differential is otherwise held up to the engine by the engine mount brackets that bolt to the block - so you have to have an Eagle block to get the provisions to bold the pinion support up - or fabricate something really stout. 

I can't speak for a Gremlin, but I did do that conversion on a Spirit!!

MacGyver wrote:   I still don't understand your issue with the seats.... That was the easiest part of the whole conversion!!!   Lopped off a bracket and the seats fit perfect!!     Not sure that any of this is actually helping the OP... It WAS about a Gremlin...     Sorry about the hijack!!

Don't forget the original post was NOT about Spirit, which is decidedly different than Hornet and Gremlin of the early years for each.

So, you are running with only three anchor points?

I know about Eagle - and their seats - it's sort of my specialty and has been for years.......... and I have done a detailed comparison on the seats. The bracket or mounting height is different due to different floors, the lower frames are different, BUT, the rest is mostly the same. For example, you can take the seat backs off an Eagle seat and put on a Spirit seat or take the Spirit seat BACK off and put it on an Eagle seat - assuming you are not trying to mix reclining and base model seats.

The mounting point you likely cut off is shown here - it mounts up inside the seat lower frame. So unless you put in provisions up inside the frame, you are using only three of the four mounting points. Doable but personally it's not where I'd go. But maybe you did mount a bracket up inside the seat frame to anchor it to the tunnel.

http://theamcforum.com/forum/eagle-sx-4-seat-comparison_topic21931.html

The basic engineering of the Hornet/Concord/Eagle (and likewise Gremlin/Spirit/SX/4) chassis/body is pretty much the same. Not identical, but similar enough it's usually not hard to make things work from one to another, with a few exceptions. There were some differences over the years and where things bolt on changed some, but with some modifications you can take most parts from one to another. NOT usually direct bolt-ons, though they are in some cases. Many small, incremental engineering changes as TomJ pointed out. You'll find those changes when you try to bolt something like a 74 front clip to a 71 body. The filler panels (like between radiator support and fenders) and many of the screw holes will have changed, but with a little effort you can make it all work. Not something you want to do for a restored car (again, with some exceptions), but for a driver where you're not concerned so much with being factory correct it's not such a big deal.