knocking mustang 5.0

[5150coupe]

i have an 89 coupe and just installed a new set of gt40 heads, e303 cam, and 1.7 crane rockers. when starting the car up for the first time there was a loud knocking noise. i thought the valves hit the piston since it was pretty loud, but the numbers show plenty of room for this combo. anyhow, i took one head off and found that everything looks fine and that there are no signs of the valves hitting. i put everything back together and still get a knocking. other mods are c&l 76 mm maf, bbk 70 mm tb, tf track heat upper and lower, and heads are port matched on both the intake and exhaust. i bought spark plugs based on a '93 cobra. even when i retard the timing it doesn't seem to have a possitive effect. any ideas?