You need to decide if you are married to an engine size or a power number. The power isn't hard to get. The parts are available. But screwing with the notion that you NEED a 396 is stretching it.

If you want 420 HP, get a small block. It'll take weight off the front end and make just as much power. if you want a big block, forget about mileage. Get a 454 or 502. Parts are plentiful and reasonably priced. Will get you get 20 MPG? Are you you kidding??? But you'll make 450 HP to 500 HP almost without trying.

Another route to go if you don't fear the computer would be a GM Gen III small block. These can be bored and stroke to various sizes depending on your budget. I'm starting with a 6.0L (364 CI) truck block, and using a stroker crank for 402 CI. Normally asperated, the engines can put out well over 400 HP to the 550 HP. And the great part is that they will return reasonable mileage when matched with a decent gear and overdrive transmission.