have a battery laying around that has twice the cca, 400 vs 800, will this hurt my car?

No will not hurt at all if it fits. cca is just a rating of output power capacity, does not make any difference in other ways.
Most high cca batteries just have larger plates in them then the lesser ones.

As dodgerunner said, no problem. Although it might weigh a bit more, make sure your battery tray isn't too rusty to hold it..

(unless you've moved your battery up front..)

I've got an MT-78 sized Interstate battery in my 86 winter beater, it's a beast, could turn over a locomotive in the dead of winter. I don't have a plastic battery shield so size wasn't an issue to me, just give me cold cranking amps for these frozen Detroit tundra winters, I'm good.

not to mention that you're going to like the higher CCA battery a lot more on those cold winter mornings.