Technically yes it is against the law, but if the truck will never be used on public roads you can do whatever you want.
But just a FYI, removing the cats on a Disco will not get you any more power, nor will a big open exhaust.
Its just the nature of the beast.

 

If this truck will never be on the road again or tagged, you can. If it is registered anywhere, it is illegal. If you do not have cats, it will run rich and carbon the heads. The Magnaflow replacements will give you all of the flow you are looking for while still retaining your stock setup and staying legal. What are the cats in the way of? Where are you racing this thing?

 

We chopped the frame and made a rock buggy
now I need help finding the gasket below the exhaust manifold
this where the manifold meets the exhaust pipe
all the local auto parts store do not have it