When I built my 4 seat buggy, I spent a TON of time trying to decide where to put the seats. I wanted to make sure adults could fit in the back without being too crunched. Leaning them back helped a lot.
My front seats are adjustable. I welded a tab on the top of my cross frame tube for the front mount and welded a piece of flat steel across the back level with the floor for the back of the Front seat mounts. This put the front of the seats approximately 1 1/2 higher than the back seats.
My rear seats are leaning even further back. I bent a piece of 1 inch tubing that went across the entire buggy and then added steel runners from the tube to the rear torsion housing. Then I drilled holes in the steel runners to mount my rear seats.
Raising up the rear seats allowed for more leg room and leaning them back allowed for more head room..