I remember my '66 Buick Electra had a big back seat--if you put something across the "hump" ant the rear floor,it was possible to stretch out diagonally and only have to bend your knees a bit to lie down in the seat..
But I found the trunk to be even bigger--I remember one night I got a bit too wasted at a party and there was no place for me to sleep at the house the party was at,so I opened the trunk and put the spare tire in the back seat and laid down in it--and was careful not to let the latch lock when I pulled the trunk lid down,I put a rag over the latch..
I actually "slept" in the trunk for several hours--it was about 1-2 am when I got in it,and the sun was up when I awoke,with a wicked hangover/headache..my friends sister happened to walk by my car and I heard her talking,I opened the trunk lid and sat up,and it scared the living

out of her and her friend !..they screamed so loud my friend's father came running out of the house in his underwear to see why they had screamed...I thought it was funny--but he wasn't very amused,neither was his daughter and her friend..