In 1974, there was no standard on freewheel removers. Every company had their own design, or 2 or 3 designs. So you need to know the brand (Atom, Malliard, Regina, ...) of freewheel to get the right remover. There are splines, 2-prong, and 4-prong versions of removers. The standard Shimano remover will not work. If the freewheel sounds and works normally, you can simply drip some oil in without removing it. But to repack the hub, you have to get the freewheel off.
I wouldn't try to respace the frame. If you buy that wheelset from Harris, ask if they can respace the rear hub to 120 mm. It involves replacing spacers on the axle, shortening the axle, and redishing (they'd probably charge for redishing). Or I could do that for you (I live near Harris).
The biggest issue here is whether you can really get the freewheel off. They can become really stuck when they've been on many years. And standard factory practice in those days didn't involve greasing the threads before screwing on the freewheel. Use a bench vise if you have one. If spokes start breaking before the freewheel comes off, you're screwed. If the previous owner never rode hard or mashed up hills and there's minimal corrosion, it may come off OK.

