79pmooney

The derailleur doesn't care what the chain is. The freewheel/cassette does. (I've used Cyclone GTs from roughly the same era on 9-speed setups. (Probably a little later as it had the reach for a 130 mm dropout spacing and the wide 8/9-speed cassette and the early Cyclones did not, but as far as the chain width running over the pulleys, derailleur don't care. And if they did, you could just go out and buy modern pulleys.

Edit: the new chain may not run well over the old cogs. If that is the case, you may need to replace the cogs or freewheel as a whole. (You will need a freewheel remover specific to your freewheel to remove it. See Sheldon Brown. They new freewheel will almost certainly take a different remover.)

Also chains are 0.5" pitch. I assume that was a typo. (ShHimano did for a brief time make a smaller pitch chain that might have been 0.4"'as a lighter racing idea but it never took off.)

Ben