If you think a Chevy engine is going to be smoother than a Caddy engine, you're kidding yourself.
There's a reason Cadillacs are twice the price of Chevs.
The fuel economy is going to suck, no matter what engine is in it when you're lugging a car of that size around.
I've driven a couple of early 60's Caddies and you can't even tell if the 390 cubic inch Caddy is running or not as they're so smooth and on top of that, they will pull as good a burn out as anything.
Also, nothing lasts forever, at some stage down the track, you'll end up selling the car. A Cadillac buyer will lift the bonnet, take one look at the Chevy engine and say "no thanks".
Just my view.