I always put my youngest in the middle of the back seat. I had a Sigma when DD1 was little & there was no way it would've fit anywhere else. Even in that smaller car I made sure there was a gap between that & the front seats.
I also needed an extension strap when we bought a 4x4. But I didn't expect the seat to come with one. They're at any big w/kmart etc. These days seats come with a lot more room in the straps than they ever did before!
I found it a lot easier to get them in & out from the middle seat too. Just because I could sit next to them & see where I was putting them