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You can wake up your 400 with a 4bbl intake. Essentually, you have 2bbl Clevland heads which breath very well. Don't be tricked in swapping to 4bbl heads as you will have a very hard time finding an intake. Besides, ports are too big anyway. The 400 is a good engine, bore and stroke wise. I would also look into a mild cam, but watch out for mileage. To swap to a 302 or 351 Windsor, the tranny will have to be swapped out as well. If you got the bucks, you can bolt on a 429 or 460 with aftermarket engine mounts.