You need to remove the crossover pipe.
You need to find out why it loosened.
Most likely bad hardware.
Nut and stud are PC 10.9. If you use a washer it needs to be a through
hardened, rated flat washer.
You need to remove the bracket and inspect for cracking.
This is a good place for an all metal prevailing torque lock nut or the
proper nut and loc-tite.
The bolt on the bottom of the alternator also has to be properly tightened.
If you can adjust the belt without loosening this nut it too loose.

Dennis

>From: John Lauterbach <jhlauterbach@BELLSOUTH.NET>
>Reply-To: John Lauterbach <jhlauterbach@BELLSOUTH.NET>
>To: vanagon@GERRY.VANAGON.COM
>Subject: How to tighten loose alternator bracket?
>Date: Fri, 1 Aug 2008 09:21:10 -0400
>
>When I tracked down sound of loose belt, I found that lower alternator
>bolt had backed out enough to cause belt to slip.  What wrench/approach
>do I need to get bolt tightened without need to remove crossover pipe?
>
>John
>Macon, GA
>'84 7-passenger