black kestrel,
I appreciate your interest in Treks. In 1984, I worked at a LBS that sold these bikes and they are well made machines. The 400 was a nice entry level road bike with good components and a nice frame made with straight-gauge steel tubes. Of course, the 600 series bikes were targeted to a more serious buyer. They were Reynolds 531 in '84 - I don't remember when Trek switched to TrueTemper for the 600 series - a much nicer, double-butted frame. The 660 model is the road racing frame and you will find it to be very responsive and a good hill climber.

Taking the money out of the consideration, the 660 is the bike I'd personally prefer between those two due to it's responsive design. (I'm also assuming that they were still made in USA in 1989.) If you want a more forgiving frame (and more cash left in your pocket), consider the 400 for a good all-around road bike.

Also, if you are still interested in Treks, but don't get either of these, look out for the 560 model, which offered nice components and a Reynolds 501 straight gauge frame. These were very nice bikes offered at a very reasonable price when new.

Best Wishes!