Better to just do it. Any number of things can happen......none of them good. An inline 300ANL (if you are planning on 1/0 gauge) can be bought on ebay or KnuKonceptz for under $30. More than worth it.
Could a 1/0 be safe with 120ANL?
the original battery to alt wire isn't fused if that helps you but than again they built your whole car so your wiring/setup probably won't be as good as there.