I have many quite old, varied, trusted 404 links pointing to our site.Is it OK for SEO to use these as inbound links for a new domain?  I would make them into 301 redirects.
Converting old 404 links to inbound links for SEO with 301 redirects
seo;redirects;link juice
My answer is 'yes'. It is OK. I've done it myself.You may even find Google still has the old 404 as a search entry in its database. I've known Google resurrect entries I thought were long gone - 'ancient' in web-land. If the old SERPs entries pop out when you replace them with a 301, then you may find you have both the old URL and the new in the SERPs for a time until Google completely digests there's now a 301 in place. I've observed this happen regularly.As pointed out - if you want any SEO value out of the old link, then don't stray too far off the original content. Think: 'transition' when creating the new content. That way, you may well benefit from any SEO 'value' the old page had.