HP estimate

You might want to think about the pistons again. Get something that will get it up to the 10.5 to 1 area with the closed chamber heads. The heads should have 2.02 intakes and as big an exhaust valve as you can get. A little porting and the heads are good to go. The cam should have enough overlap to bleed off some pressure and you are good to go, I had a 367 that was similar to your build except with iron heads and a comp 268H cam that produced a little over 400hp so your 408 with a little bigger cam should get close to 500hp. Make sure you do not under fuel it, it will like a lot. The timing on my 360 was 16 initial and 34 total and I ran 91 octane. It could run as low as 87 but it didn't really like it.

Jack