Thank you for your help. I was able to find drain plug but it's so stuck that I stripped the head and wasn't able to remove it, I just left it alone for now.
As far as flush goes I watched some Chris Fix YouTube videos on him using Prestone flush + cleaner and he was driving around for a few days and then he had nasty looking brown coolant to drain. Well, I did the same thing and after few days driving my coolant still was a transparent light iced tea color. After draining and flushing everything well I thought I'm ready and took that water pump out and found out this inside.

(this photo is sideways for some reason, there are no loose particles or rust, all that is stuck pretty hard to the block walls)
Not sure what else can I do. Is it normal for 50 year old engine to look inside like that? Should I just install new WP add 1 gallon of concentrated coolant and top it off with distilled water or should I do some more cleaning?