Guess my 273 hp

Exhaust manifolds are not limiting your RPM, I've never run headers and my 273 would pull past 6K anytime. Original AFB, Holley 600, and especially a TQ. I've run Duals with Turbo Mufflers and the straight thru Commando single exhaust. I'm sure it will make more power with headers but you don't need them to pull past 6K. I'd look elsewhere. Ignition, Carbs, Camshaft / Valvetrain?