Welcome to the forum!
Well if it were me I would pass on the 396 build and use your existing heads with a 454 block and save the date/code correct 396 block, they aren't getting any easier to find and 454 blocks can be had with ease.
500HP with pump gas using a 454 block wouldnt be a problem.
I'm not a cam guy but I'm gonna throw this out there anyway ( be easy on me guys)
Healthy bumpstick with around 550ish lift - 290-300 duration, about 10.5 to 11-1 compression, good single plane intake, 850 Holley double pumper carb, with some good "worked" oval port heads, that should get you real close.
Cp