I want to fill a cake with whipped ganache and the frost it with the same ganache. Will the ganache goosh out the sides where it is filled or is it strong enough to stay put if I make a dam?

When you whip ganache it gets pretty stiff so you shouldn't have to worry about gooshing. I pretty much always use a very thick dam just to be sure though. I would reccomend just using set ganache for the frosting, and whip the filling, sometimes when the whipped gets stiff it's a major PITA to spread.

When I whip ganache for filling, I use the 2 chocolate :1 heavy cream ratio, rather than the 1:1. Good luck.

I've used ganache as filling many times. I have a cake this weekend that will have ganache filling. I let the ganache begin to set a good bit, make a frosting well and then fill with ganache. I put that layer in frig to set before adding the next cake layer. Never have had any problems. It sets
up fine and is delicious. You can use same ganache to pour over cake, just don't let that set as much before pouring.