My first question is what lead the OP to think the bike was a Nishiki?

The bicycles vintage can be determined by applying the information contained in How Old Is My Bike?

As for what brand is the bike, there is precious little to go on. Good clear close up pictures of the drops would add a bit of help. Also, measure the diameter of the seat tube (measure accurately and that number will help to determine tubing type) and the width of the bottom bracket housing (70mm = Italian or sometimes Mexican while 68mm = just about everything else).

Other than that, based on the information provided, I can offer little more. I would say that the Shimano 600 stuff is first generation, putting the bikes vintage around the mid seventies, approximately.