You could actually get a 425hp 409 in 63. I had one in a 58 impala with a Muncie 4 behind it, 2 Carter AFBs and would put valve rockers through the valve cover if you overwound them. that was a fast car, but would break all kinds of parts. I used to have 2 valve covers on the garage wall with holes from the rockers. The motor came from a 63SS that had been wrecked.