An explanation of isolated ground vs case ground alternators

With Non-isolated ground alternators the case of the alternator is connected to the negative and so the engine block becomes a parallel current path.  This can produce voltage drop in the engine block and so is more prone to corrosion issues.  Non isolated ground alternators tend to have better cooling because the diodes can use the case as a heat sink.  Even though the case is connected to the engine block you will still need to use a dedicated negative wire to take the current.  This is because the connection to the engine may not be that great,  The photo below shows the shiny unpainted area of the alternator mounting foot that can be used to connect a negative battery cable which will be fastened in place by the alternator mounting foot.

To tell if you have an isolated ground engine you can look at where the battery cables come to the engine.  If they come to an isolated post on the starter motor then you have an isolated ground engine.  The senders for oil pressure etc. will have two wires instead of one.  If you have a non-isolated ground engine the oil pressure sender will only have one wire and the negative battery cable will go directly to the engine block.

If you have a non-isolated engine there is no advantage to getting an isolated ground alternator but it wont hurt in any way.