I don't get why anyone would pay for those logs. They are minimally better than the stock manifolds. Get shorty headers if you are not getting kooks.
Long tubes are the way to go, but if price or clearance is an issue..mid lengths. Shortys seem like a waste of money. Just get an electronic cutout if you can't get REAL headers.
Stock exhaust manifolds can only flow so much.
I am not positive and someone correct me if I am wrong, but Cobra manifolds are good for over 600 HP. Do you intend to be making that much power anytime soon? I know I won't.
I am assuming that you are new to the Mark VIII world