I have a client with a large discussion board which Google isn't seeing the content (8k pages). I suspect we need a new discussion board/forum software OR a fix to his ancient ultimate BB software.

Does anyone have recommendations on the most Google friendly system to port over to?

I'm no SEO pro but a follow-up marketing guy needing some technical help...soon.

VBulletin is really good... but you need to make sure whatever you use has SEO plugins - you need to have different title and descriptions for the threads, you need to have people linking to certain discussions etc etc

Is it a members site? are the pages only accessible if you are signed in or can you read and have to sign in to post? The first would stop the spiders.

Also you should have the site listed in Webmaster Tools ay Google? It will then spider the site

Not a members site. People must login to comment but anyone can view.

We are on ultimateBB which is ancient. The forums don't have Google analytics but the rest of site does and webmaster tools... I think that the site has a huge amount of valuable content and very poor structure so it isn't ranking well on Google. I'd like to find someone with the SEO/site structure skills to diagnose my problem and verify solution. As a complete novice, it seems clear to me but who knows.

I did a test looking on Google with "site: url and search string" and Google finds the search string in Windows/firefox and Apple Safari but the url Google reports back when clicked shows "page not found". I conclude: Google knows of the content. When I do a Google search, Google doesn't seem to be finding our "search string".

Am I thinking right about this?

Although we have 8k pages, it looks like Google is indexing 800 pages. I was using Hubspot and in a site analysis it reports that Google has 800 indexed pages.

I'm theorizing that Google isn't indexing the forum conversations. I know there aren't Title, H1, meta or keywords being extracted and placed into posting pages so we are leaving alot of Google love on the table.

It seems like migrating this content to a better Google friendly structure will give the site more Google Love. Am I thinking right?

More thank likely.
You want to be sure that those pages are being crawled right to begin with (are you using an xml sitemap?), and then also that the login isn't stopping the crawl like Aussie already noted.
Next, you are right, and each page - if you want Google to really count it needs to have it's own Page Title, individual Meta, etc.
Good chance this is your problem, or at least part of the problem, depending on any other roadblocks that might be in place...