Changing from 20 to 40 amp alternator on JD 455

[caseyvand]

I'm planning to change from a 20 amp alternator to a 40 amp alternator on my JD 455 . Physically the change appears to be no problem but the existing wiring to the 20 amp unit only has a 2 wire plug going to it. The 40 amp unit will accept the existing plug ok but it also has a threaded stud beside the plug which I think (but not sure) has to be wired to the Positive terminal on the batery. Has anyone made the same change .......or know......if a different wiring setup is required.
Thanks.....Casey