In short, If you are running the Z loom then you should run the Z alternator.
There are two very seperate systems at play here, that being the charging system and the battery system.
The battery provides power to everything...that being said if the alternator goes down then the engine will still run for a while...just off the battery.
But once the battery is dead she will stop. So all you have to make sure of is that the alternator can keep up with the charge being drawn from the battery.
That is ensure the battery is provided with enough current to charge the battery in respect to the drain..ie head lights etc.
To my point.....if nothing else has changed (i.e: no spotlights or sub woofers) then the current (pardon the pun) alternator should do the trick.....if it physically fits.
A Six cyl to an eight cyl is not a big change...besides the spark works off voltage not amps.