Help! Century SmartFit Plus and 97 Toyota Corolla

We received a Century SmartFit Plus (not the 22V) for a baby shower gift and tried to install it in our 97 Corolla. Well, we tried to install it as best we can and the seat can move from side to side more than an inch!! I mean it doesn't feel safe to me! (It's not rock-solid-unbudgable!!!) Are we doing something wrong? My friend has a Graco infant seat (the one that comes with the travel system) and hers fits snugly in her 95 Corolla. Does anyone know if the century SFP is incompatible with the Corolla? I compared the 'budgability' of the the SFP and the Britax RA and yikes, the SFP seemed unsafe compared to the unmovable Britax RA!!

Anyone has the same problem, or has installed the SFP successfully on a Corolla? Now I'm considering getting the Snugride!! Or should I just go straight for the Britax RA instead? (baby due in 3 weeks).

Any advice is appreciated!