Check out this link for successful 3-across combos in a Prius. In every scenario, at least one Radian was used. Here's a thread about successful Radian installations in Priuses (Prii?
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You mentioned that your seat was already moved up quite a ways for the Britax. For an older baby/toddler, you can install the seat as upright as 30*. I don't know how a Prius compares to a Corolla in terms of roominess, but I have lots of room in our older Corolla even with a Boulevard RF in back of me. It's at a 30-35* angle, and there's a decent gap between the car seat and my front seat. The Radian has a taller shell than the Britax seats, and it tends to be more difficult to install upright. It will likely take up more room than your Britax.
You might also consider looking at the Complete Air if the Radian doesn't work out. It is slightly wider (about 18"), but it is a low-profile seat like the Radian is. They usually carry it at Babies-R-Us. Since they let you take seats out to your car to try them, you could see if it puzzles well with the KeyFit (which they also have at almost any BRU). It is MUCH easier to get a compact, upright installation of a Complete Air. It has a slightly taller shell than the Radian for lots more years of RF!
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