Going with 2x4's is way cheaper than the 3x2's. Especially if you're handy at carb tuning, then you can start with nearly new second hand std 4 bbl Holleys. Two 390's on a 2x4 is what I'd do with a 390. Maybe even the 450 carbs. Twin 600's is better matched to more cubes, they ran great on my old 427 when it was stroked and poked to 454.