You're probably going to need new heads for 400-450 HP (is that goal at the flywheel or at the rear wheels?), particularly for the 450 HP number. The cast iron GM Small-Port Vortec Bowtie heads can make that kind of power at the flywheel. If you prefer aluminum, there's the GM Fast Burn heads. Both those are tapped for both the Vortec intake manifold and the earlier SBC intake. There are other aftermarket heads available.

https://www.gmperformancemotor.com/category/SB145.html

You will need a more aggressive cam to meet your goal. Maybe you could even hit the 400 HP @ the flywheel with the base Vortec heads. You have to select the heads and cam as a package, so the duration and lift takes advantage of the head flow characteristics. I'm not up on selecting a cam for a carb setup. For the last 25 years I've been dealing with the fuel injected LT1.

Unfortunately, the number of users on this site with 3rd Gens has really declined. Seems the only active area is the few die hard LT1 users. Every once in a while a veteran 3rd Gen owners will show up and look through the posts on this particular forum.