Thanks. These are good things to consider. I suppose in my case I'm primarily wanting to just find a new home for the old content since eventually I will just get rid of those websites and domain names. And then if it adds a bonus to the new website, then great!
The old content is automotive, diesel truck related, which the new site is too. The difference is the brands. So I think the old readers would just like the content to be live and not be too worried about where it's hosted. And the new readership would probably find it all interesting as well...thanks for helping me think it through!