Hi just put the back seat together and I have a large 1 inch gap between the back seat bottom and the back upright part. The center hump area meets the back part but left and right is off by a large amount. The seat is in fairly good condition. Could the foam be that compressed on the seat bottom part? Nothing bent and yes it is in the hooks at the bottom and back upright part feels in place.

I assume you just got it back from the upholstery shop? My guess is he did not put enough padding on either the back, bottom or both. I just did mine and I only have about 1/4" gap right in the middle 4" or so. Now after thinking about it, It could be TOO MUCH padding at the front of the bottom seat, too. If you put a good layer of felt on the front rail of the spring bottom, and put too many layers of foam/cotton, then you might make the material to short at the back of the bottom and it would have a tendency to draw it down too short when you pulled it tight.

That gap is not too bad...I have seen worse. If you used original foam and didn't add anything than this is within the tolerance range i have seen. If you wanted to fix it, you'd really need to add a bit of foam to each the backrest and the seat, as trying to make up an inch on only one part will make the cover very tight.

Is you backrest down all the way? Sometimes it gets hung up a bit high.

My 70 Cuda has the original back seat in it,and there is a pretty good size gap on both ends.

The factory knew there were big gaps around the back seat bottom. On orignal cars you will see the factory painted blackout in the corners. So you will see black instead of body color.

What do the pictures of your car look like in that area after you took the seat out.

Thanks for these replies. As long as an inch or so gap is normal, I'm ok with that. I might try to add some padding in the future, for now I need to continue with the restore. No I did not have this seat reupholstered. I don't think there was any blackout back there but I will check the old photos when removed.
Thanks again guys.

My 71 challenger developed the gap after the new seat covers were put on. With the original covers there was no gap there. After I had the new covers put on over the original foam, there is now a one inch gap along the entire length where the back meets the bottom. I just leave the seatbelts tucked in there to hide it.

same problem with my cuda convertible after new interior seats, solved it by pushing the seat back further and locking it in the front behind the bracket..seems to work for me..

I had the same gap on mine. After pondering it for about a year, my thinking was that the seat bottom had settled or sagged from use over 40 years. So I removed the seat bottom and tweeked the frame to eliminate the gap.
Looks great ever since.

This is a picture of a 71 Challenger that 68HemiCharger was doing some work on. Notice the factory black out on the ends of the rear seats where the seat and rear panel come together.

The amount and even the number of places blackout was put in the interior varies per car. But if you're looking at a restored E-body with a bright colored paint, look in the rear seat and you'll notice if they put black out on it for not.

I struggled with this for a couple of hours and the best improvement was bending the lower rear seat tabs(bolted on bottom) toward the trunk. Now just have slit gap on one side which is much better than the huge gap which I started with.

Never been a big fan of upholstery shops approach to restoring seat assemblies. You are at the mercy of their process which does not always mirror the original assembly. I since have learned to do them on my own. I always use new foam, jute, burlap etc. Frames checked and verified OK.. fixing whatever needs fixing or straightening. I put material back on the way it came off and I typically end up with good results. Here are some e-body seats I finished not to long ago.