Not a lot of space - took a couple of photos of my 65 Barracuda. I was going to use a set of 65 A Body buckets with a buddy seat in my 65 Dart wagon and there's not enough room for the buddy seat. 66 Buckets are not nearly as wide and the buddy seat fits with room to spare.
Years ago I tried squeezing a later A console in my 65 GT and got it to fit but it was tight up against the seat frame and being so tight, I couldn't move the seats back and forth. Guess it wouldn't matter if you didn't need to move the seats.
I confirmed with Legendary that the 66/67 seat bottoms were quite a bit narrower than the 65 and earlier seats. I had a set of 66 cores and they're about 3 inches narrower. The 68 B Body buddy seat I have fits in-between the 66 seats with room to spare. No pictures as I have everything out for re-upholstering.