Having owned a '60, '62, '63, '64, '65, '66, '67, '70 and a '72 - I can say that my memory always seems to point towards my '60 as having the most room. I'm not 100% sure, but that's what I remember. Then again, i was 80 pounds lighter in 1980 when I owned that one .. LOL.
Head room yes but there is still the issue of leg room.
Cadillac sells a Chevy sedan (they call it a Cadillac but it is a Chevy sedan if it has a SBC under the hood that was sawn in half on the drawing board and 14 additional inches added into the passenger compartment (all of it was behind the front seat though, so as far as the driver is concerned it is still a Chevy). It is like adding a leaf to your dinning room table, same table just got more room to pig out if you can beat the inlaws to the food!
Happy Thanks Giving to everyone (even those reading it years latter give thanks we are still here).
I say any 4 door full size Impala or Caprice has more room then a 2 door! More leg room more room to get in and out also the Caprice and the Impala Custom has that sunk in rear window from 1969 to 1976. But most 2 door cars has less leg room.
My 68 is pretty big on the inside but i think my 74 caprice is the biggest car i and all of my friends have ever been in lol!! Even my little 2 year old niece said my car was huge the first time she sat in the back seat She just looked around in awe