HowTo? Fixed / Singlespeed conversion from an old bike

Is it defintely a freewheel hub not a freehub?

yes, the bike is from the 1970s.

Any comments on chainline / where to fit the chainring on the crank?

trial and error, most likely fit the chainring on the inside of the crank spider.

Does OP want to go fixed, or at least wants to try it?

judging from the first post, no.

Is it worth getting new wheels with a flip-flop hub?

not unless the op wants to go fixed.