I have a theory on what may be wrong.
Did the intake gasket look like this or were both water passages open on the ends?
Gaskets for earlier intakes prior to the tbi engines had both large water passages open and it didnt matter which way they were installed because the back side of intake was blocked off.
On the tbi intake there is a water passage that is open but coolant is forced to flow from front to back when proper gaskets are used.
This gasket with one end blocked off must be used (blocked portion to the rear). If the wrong gasket is installed coolant pretty much bypasses the head and chambers will run too hot creating detonation problem.
Dont ask me how I know!
Also check your ignition timing. Computer should be able to pull back timing if its too advanced though.