390 vs 408

The difference in the 18 Cubes isn't going to be that much of a deal, it's the shrouding of the smaller cylinder bore. I would say, you will be maybe 20HP less?

Pro's of a 390?

IMO, less piston surface=less chance of detonation.
You have the 318 already=you know the block is good
I believe, and always have believed the 318 block is stronger than the 360. Say what you will, I have never had a major 318 failure (except for my first go around, a set of KB pistons for my 390, I love my ROSS pistons...), and LOTS of 360 failures. Fluke or fact, thats my OPNION.
I have over 40K on a 390, runs at WOT a ton, and has been flawless as far as internals after the KB's........ I love my 390.

I also have experienced better MPG's with smaller bore engines.

I think the 600 will be more than enough for the little more you will spend on pistons, and the peace of mind knowing you started with an engine that you are familiar with.

My 390, is built for 3500RPM, WOT power. It even has ported small port 302 heads. It is a truck motor, so I keep it on the low RPM, it will zing past 5k, and build power too. It get 12-17mpg, in a dually pickup, with 4:10's.

Pulls the hills better than the 413 dually I used to have, for sure.

Pro's of a 408?
Well........

A little more breathing, a little more cubes, and you can say, "408"..........