The Britax seats have built in lock offs, so you don' need a lockiing clip and NEVER EVER EVER use a Mighty Tite belt tightener. They overstretch the belt.
The middle belt should have a locking latchplate, meaning once you buckle it and tighten it, grab the lap portion ad pull, it shouldn't come out because the latchplate locks on the back pull.
The middle is the safest, but only if you can get a good install. If you cannot, install where you get the best fit. The Britax seats CAN brace meaning rear facing it can touch the front seats. I can fit a Britax Marathon rear facing in our Chevy Aveo subcompact without having to move the drivers seat for my 6'3" husband, you just have ot know how to install properly. NOT saying you don't, but with practice you can get it in without moving the seat.
If your child is over 6mos old you can have a more upright install, it doesn't need a full newborn recline. That will help it fit better too.
the Evenflo Symphony will be just as tall if not taller as the Boulevard. The Britax seats are crazy easy to install, especially with hte build in lock offs. PLEASE find a CPST in your area via www.safekids.org to help you install the seat you choose.