° If that was the original rear derailleur... I know of two combinations which that model definitely used during the late 1970s:

1.) SunTour V-GT Luxe rear derailleur ~ 5-spd freewheel (14-32 cogs) ~ 40-52 chainrings

2.) VX-CT rear derailleur ~ 5-spd (14-32) ~ 36-44-52 chainrings.

These both were matched with Suntour downtube ratchet (friction) shifters. There "were" some indexed 5-spd and 6-spd thumb-shifters available later, but these may be VERY difficult to find now.

Most modern "touring" bikes I've seen basically use mountain bike rear derailleurs. Any modern MTB derailleurs can be used with just friction shifters as well as with their intended indexed shifters. And, even the cheaper models among Shimano's broad range of MTB derailleurs will probably shift better than most older derailleurs did.

Buy a new chain too!

I commonly run 8-speed chains (preferrably SRAM) with my old 5-speed components and they always seem to work fine. An 8-spd chain will easily glide through pretty modern rear derailleurs and your local bike shop may even have some perfectly good "demo" or very lightly used derailleurs which were simply changed out for a customer who wanted something more fancy. These are usually sold really cheap - just so the shop is not stuck with dead stock.

I would suggest you replace the freewheel, or at least look for one, now. They do wear eventually, and 5-speed freewheels are becoming more difficult to find all the time. IF you can still find one with a comparable range, a simple 5-spd replacement freewheel will save you from having to change the rear spacing of the frame to fit a longer rear axle - which a greater number of rear sprockets would require.

Perhaps someone else can tell you specifically what you should definitely NOT use. But, you really can get by VERY cheaply.