As long as the whole base is touching the seat bottom and the back of the carseat is touching the back of the seat there shouldn't be a problem.
See, that right there was the problem, the bases of the Marathon and the Wizard were not wholly on the seat. The very back of the bases were about an inch off the seat, whereas the front of the bases were dug wwwaaayyy into the seat. The backs of the car seats were touching the back of the seat. It was either install them that way or end up breaking the back window glass if I put them in the "right" way. The tops of the car seat backs would press into the back window a LOT if I tried to put the seats in less upright and with the bases flat on the seat. Here is a quick illustration - the first one in red is how I couldn't install them flat on the seat, and the one in blue is how I ended up putting them.
You can see by the one in red that if I had tried to install the seats normally, that tightening the belt would have pushed the tops of the seats right into the window more.
Their truck is a 2001 F250 Super Duty diesel with only an extended cab. I did not find any tethers, and Grampa told me the seat back was attached and could not come forward at all and I didn't see a way to make it come forward, but he did say the seat bottom would fold up.