While I've always been fond of FE engines, I'd still think the 400 is the better way to go. The parts are cheaper, and you can make a reliable 400 with good power for relatively low dollars. A good cam, a straight up timing kit, a 4bbl carb (e.g.: Holley 650) and some work on the exhaust (i.e.: headers, new muffler, duals, etc.) should wake things up a bit.

Don't get me wrong.....the 390 is a good engine, but they're getting a little more scarce now, and the 400 is plentiful and cheap for the potential you can get out of it.