425HP is not at all difficult with stock manifolds, though I'd port-match them just because I can't leave anything alone or pass up "free" power. You don't need a Holley or headers to hit your target; I've done it.
What's your budget for the motor? Do you want it to have nice street manners and/or relatively smooth idle?
The stroker deal will be less peaky at the same power level, since as a street motor what matters most is torque; if you need to buy all the parts anyway, it's a slam dunk. I'd go with the Stealths, with upgraded hardware and attention paid to the valve job & chasing threads. If you want to run 13/16-hex plugs, you need to open up the area where your plug socket interferes with the casting.