Identify the hub to make sure it isn't something totally obsolete like a Maillard Helicomatic.
In my limited experience with 5 speeds, expect to have to add a washer or 2 under the DS lock nut to give enough cog clearance to the drop out.
This throws off the dish by 1/2 the spacer thickness.

Does the RDER have enough capacity?
Largest cog-
Chain wrap-

Since age is permanent, you might consider a crank set with smaller rings.