Thanks Jake and all who replied.  I've got enough here to investigate for a few days when I get the time.
I had found and cleaned the first two grounds - battery and transmission - where in the engine compartment is the engine ground?

Date: Thu, 2 Dec 2010 21:17:38 -0800
Subject: Re: 91 Vanagon Manual - slow cranking....
From: crescentbeachguitar@gmail.com
To: rueffy@hotmail.com
CC: vanagon@gerry.vanagon.com

Hey Brett, you need to clean all three grounds - the one in the battery box, the one at the front of the transmission and the one in the engine compartment.
I never had any electrical problems or slow starting but when I did my Subaru transplant I ended up adding a second engine ground and a second transaxle ground, just because the straps were attached to the engine and trans respectively.
Anyway, my old 2.1 WBX starter whirls the 2.5 litre engine plenty quick, and everything else electrical works better, faster brighter.
Jake