Do you plan on competing competitively ? Doubt most the top guys are dual purpose. and if your fine with a less competitive car you could dial it back, handling matters more than power, especially in a parking lot autocross no long straight aways, lot of power that's a lot work and experience to get it to hook.
1st I'd forget about top end hp, what are your trans gear ratios what gears your gonna spend most of your time in, probably only one or two of them in a autocross, figure what rpms you will be turning at the speeds at most events. Especially what rpm you will come out of the average corner. Then there's how much power you can reallistly hook out of the corners.
Say it's 3000-5600 rpms, your gonna spend more time in the bottom half of 3000-4300 rpms, thats 58% to 83%, hp% of tq, so say you build a fairly high 1.3 lbs-ft per cid at peak tq and flat in that range of 3000-4300 rpm ,
At 1.3 lbs-ft per cid a 396 = 515 tq = 58% 299 hp @ 3000 rpm to 83% 427 @ 4300 and about 486 @ 5600 rpm.
Lesser lbs-ft per cid which is where most average guy builds live will make less power and obviously more makes more, same with less or more cid, at 3000-5600 rpm.