you'll need to check the bolt-circle diameter of your crank/current chainrings. Your 400 series "exage" double is probably a 130mm bcd, if it has 52/42 rings, but double check before you buy; it may be 110mm. any rings with a 130mm bolt circle diameter will fit. Things to watch out for: make sure the rings you get are 3/32" for use with derailer systems. (not 1/8" for singlespeed) If you get ramped/pinned rings, make sure they're aligned properly for optimal shifting. (Once you get them, come back on here for help.) If you wish to change ratios, now is the time. It may be easier to source a 39t inner ring, but 42s are plenty common. Here's a fine example of a compatible ring that'd look "right" on your late 80s ride:
http://www.treefortbikes.com/eurl.ax...22346928___325

I'm guessing that your fuji is a 6 or 7 speed system; 8 speed rings shall work fine.

if your biopace rings aren't beat up too bad, pm me. I'd like to buy them.

-rob