You are correct about them BOTH being isolated ground, as the truck was built. This was done so there was only one part needed to keep in inventory. But in reality, you only need to have an isolated ground on the passenger side.
I think the stock starter brace is still available through GM. I have never needed to replace one. I know the part number has been posted before.
I think some people have made a new brace.
I am lucky, I have a good local rebuilder and I also rebuild my own.
I don't know what your problems are, but before you replace anything, I would try to follow the TM troubleshooting section(-20 I think).
Any "new to you" CUCV probably will need the alts rebuilt. They seem to die, once we get them and use them again. I have rebuilt both of mine, about 2 years ago, and they are doing well.
One more thing, search for the "doghead relay modification" and consider doing it. (best results for searching are, use the "advanced search", and use a couple key words, then select the correct forum to search, then search.