I haven't done the vinegar or swimming pool acid clean, but I have sent quite a few cast iron blocks to a company here in Australia called "Redistrip" . I imagine they use a similar acidic type of treatment, strangely enough some cast iron blocks come back in perfect condition, while others have an etched,almost sand blasted type finish on all machined surfaces...the slant six block I sent to them came back with this finish after cleaning. Perhaps the type of iron used in the casting process,together with the pretty average quality control back in the day created a lot of variables in the actual iron content. Interestingly,any higher performance type block comes back perfect,I had an older Ford Motorsport block done,also the Australian Hemi six seems OK....I'm sure the vinegar or swimming pool acid does the job,perhaps just keep an eye on the process instead of leaving in for long periods next time?
Its a bit hard to tell in the pics,but it does look like an old gasket....I would imagine the whole head would need to be thoroughly degreased and hot tanked prior to the vinegar treatment for it to really do its job?

Holy cow...never seen anything like that.

I wouldn't trust that head as far as I can throw it.