You pretty much hit it in your first response. You have the stuff, so that is a good reason to use it. If you have a 383 and think that going through a bunch of extra effort to swap to a 360 Mag will make for a much more efficient package, you'll end up not getting much reward for the effort. Really, fuel efficiency has never been a strong point with any 360 I've ever seen.

With your goals (20mpg?), I probably wouldn't go with that big overlap WL cam either.

The 383 is actually a very good all around street engine. For MPG and good horsepower, I would build it with the CC heads, a short cam with wide lsa and big exhaust split (stock Magnum cam maybe; 1.6rr would help on the power side), work the heads for flow and run tight quench. I always like your posts, PR, you seem to have a good perspective. Give some more details about your combo.
-dulcich