Headmen makes headers that fit the grand prix and other big pontiacs with the engine set low in the frame.
Dcr is the best way to solve things without using a thick head gasket or changing heads back. A much larger cam will solve some of the knocking issues. Poncho wont make power if you cant get any timing into the motor.
I am looking into a product to coat the piston tops and valve heads to help keep knocking down in my engine. Also i recently read a good article from david vizzard about controling detonation.
An aluminum intake that lets some air under it may help alot if your still running cast. But sounds like your problems are much bigger than a little air temp is going to fix.
Also change ypur thermostat to 170 if at all possible. Keep it running cool will help the most with knocking.