If the new wheelset is english-threaded for a multi-speed freewheel, then you can put a 6speed freewheel on there if the wheel is spaced at 126mm.... or if you have a suntour Ultra 6 freewheel, but those are getting scarcer as time marches on. If the wheel is spaced at 120mm, you'll want to add 6mm of spacers on the drive-side behind the jamnut, or just run a 5 speed freewheel.
If the frame is spaced at 120mm (prolly is if it's a 70s Moby GT), then you'll likely want to cold-set the frame to 126mm to accept the 6speed freewheel OR just run a 5 speed.
For real, having that one extra cog, imo, is not worth respacing frames at all, and hardly worth respacing hubs. At 126, you can go 7speed, which is slightly more worthwhile.... slightly.
If your new wheel has a cassette hub, then you'll need to run a cassette. You'll also wanna measure the O.L.D. and see what you're working with....
Now, you're messing with a French bike, so beware of oddball threads EVERYwhere, esoteric BB shells, and even spooky things like a helicomatic rear hub on the orginal wheelset. Motobecane was way more "normal" than Peugeot, so you might be OK. And, if that's an 80s frame, it's likely to be made in Asia, with more standard threads and a 126mm rear-end.
I hope this helps.
