I have an old worn out Maillard 7 speed freewheel on a 89/87 era Athena hub. I happened to check on the status of an order I had placed with a shop for a 7 Speed Sun Race 13-24 replacement freewheel, when they mentioned having a NOS Synchro 90 7 speed FW in stock. So I bought the thing, and threading issues aside, will this give me any other grief? It certainly weighs a ton, but I'm used to cassettes...

There should be no threading issues, at least as long as neither the hub nor freewheel are French threaded, and you get the thread started correctly. If you get it cross-threaded, you'll need to buy a new hub. I guess this is a little more intricate than replacing a cassette, but it's the way they have been for at least 50 years. Why do you say there are any?

The Synchro should work fine, really.

BTW, the fact that the old freewheel is made by a French company does not imply the threading is French.

Be sure you double check the travel limits of your derailleur. Despite standards, not all freewheels have the same spacings. You really don't want the chain to overrun the inner largest cog and get trapped between the freewheel and the spokes. You'll need several new spokes, maybe a lot of them.

done and done. The new freewheel works beautifully.