I'd normally say keep the 390 as I love the old vintage stuff. But it would be really cool to drop in a 4.6 DOHC modular engine and trans. As much as I like the FE engines the 4.6's are a really nice upgrade that won't break the bank. And there's more than enough room under the hood there. I really like the 4.6's and have been thinking about tinkering with one myself. And there are tons of new parts and kits with blowers or turbo's that are netting some big HP numbers.