The biggest problem with modifying your OEM exhaust is that if you phuck it up then you've got to buy a new one anyway...
Also, I don't know what the EPA is like in your neck of the woods, but if you don't have a 'legal' replacement on hand then you still have to buy a new one.....
My suggestion is to buy an aftermarket exhaust of your choice and keep the OEM in the shed just in case.....