The runners should already be port-matched to the head... porting it wider wouldn't do anything because the exhaust ports on the head would be the bottleneck in the system. You only need to port your exhaust manifold if the runners are too small for the ports in the head, such as if you port/polished the head, resulting in larger exhaust ports on the head and making larger manifold runner ports necessary.

You might as well just get aftermarket

 

just goto junkyards and look for an ex cat

 

buy a magnaflow high flow cat? might not be legal in cali tho

 

Using the Magnaflow cat should keep the CEL off. You just won't pass visual inspection if they notice it's not OEM or CARB approved.