With the lump head and 194-204 cam, the Gonkulator puts your build at Torq 257 at 3300 Powr 211 at 4700
Going to a 450 Holley adds 0 ftlb and 2 hp. In my carb shootout testing on the car at 170-180hp, the 390 and 450 Holleys ran just about the same, it was more a matter of jets, weather, etc as to which one "won". The 390 would open the secondary most of the way, the 450 would open it maybe half way. Bigger than 450 I think you'd start losing down low.
Find a cheap 600 Holley at a swap meet and try it on there, they are still $20-$30 and easy to find. That is likely TOO big but you never know with carbs til you try em on a given combo.
