cubic inches don't always mean horsepower with factory stock engines. The 400 was a really low compression smogger engine...but don't toss it aside for something else. They can be stroked...but for a street car, a stock stroke 400 with some higher compression pistons (not a lot of options!) can really make some good power...

However, shooting for your goal of 4-500 HP...you might be better off going the stroker route...a lot of stroker kits out there...