i may have an idea why ur motor popped. my last motor on the last 2 cylinders also went bad from knocking but it was #8 that blew a hole in the piston. what happend was my motor had oil blow by and the oil would come out of the PVC on top of the valve covers. if u follow the PVC line from the passenger side valve cover, it goes to the back of the upper intake manifold. well when the coil goes into the intake it dumps oil right into the intake ports for the last 2 cylinders. later this leads to a huge amount of carbon build up which will caused detenation and eventually blow the hole in the piston.
i solved my problem by pluging the factory hold on the back of the intake manifold. then i drilled a new hole on the side of the manifold where now it should spread the blow by oil more evenly.
good luck
Chris