The CFM has nothing to do with it running rich. Sure, it's a bit big for a stock engine, but jetting it right will get it running better. I had a Holley 3-barrel on a 283 with Power Pack heads and a Duntov cam that didn't run rich but you couldn't open that big trap door below 3000 RPM without a huge backfire. When it caught all hell broke loose!
The Caddy in question isn't a stocker, either.