The 400 is a good engine for torque.

You would be surprised what a difference freeing up the exhaust, adding a dual plane intake/4bbl carb, and bumping the timing up will do. Just keep in mind its a heavy car with a stock engine that doesn't rev too high, so torque is what you should go for.

Change the diff gears and you'll be giving a lot of people a good run with your boat

There used to be a local guy with a 1970 455 Buick stationwagon that got into the low 14's with just bolt ons and tuning..... It was funny watching him shut down Mustangs!