shortie headers are a pain to install for their gains, but if u live in CA they're a must, although your going to have some leak probs im guessing cuz the steel expands more than the stock cast iron and will entually become loose from swelling and then shrinking or blow a header gasket from expanding too much. Ceramic is the way to go, tho i havent heard too much probs w/ plain steel on our 4.6's
My guess is 11-13 if NPI and 13-16 PI to the ground