Some ¾- and 1-ton trucks came with Vortec heads equipped with a specially hardened exhaust seat that kills low- and mid-lift flow. Casting number 10239906 should be avoided. Look for casting number 12558062....
read this thread, it answers everything you need to know and more about Vortecs
http://67-72chevytrucks.com/vboard/s...d.php?t=485197