Swapping to a 318 I think would require computer, harness, trans, mounts I'm pretty sure. BIG job. Plus you'd be going backwards with power and mpg. The 4.7L isn't crazy powerful but I believe puts out as much or more than the 318 and gets better fuel economy while doing it. Go to carb and mpg will be even lower.
I'd swap to another 4.7L or rebuild/fix your current one, they are not bad engines at all, actually very good engines.
IF you're set on a swap, swap to a hemi. It will bolt to your current transmission. It would be an easier swap to a more powerful engine that still probably gets better mpg.
Breathing in and out is good enough for me.