Shimano made "Wide" UG (Uniglide) for 5 and 6 speeds, and also "Narrow" UG for mid 1980s 6 & 7 speeds.
In the late 1970s Suntour introduced their "Ultra-6" freewheels which would fit a 6 cogs into 120 mm. spacing, and shortly after that they made the "Ultra-7" to fit within standard 126 mm. 6-speed spacing.
But, the problem is not at all so simple as the width of the spacing between the cogs... although that too would be an issue since most older freewheel cogs used flanges at their base for spacing them between each other (and to offer more threads for locking them onto the body).
More importantly, I would think a huge issue would be the specific diameters of the individual cogs and even the direction of the threads on the cogs... and also more specifically where any given cog would have been designed to fit on one of several different diameter shoulders on a particular freewheel body. There were usually two or three different specific locations on freewheels and the sizes and the thread direction and even the thread pitch was different between Shimano or Suntour or Regina or virtually every other individual manufacturer.
When you get into the 1980s it becomes even MORE
complicated since both Suntour and Shimano were furiously experimenting with different tooth designs.
For example, in some cases a chisel-tooth Suntour cog must be placed at a specific position on a body.
I've seen some Suntour re-builds which were disastrous because the slant of the teeth tended to toss the chain off. That was because it was designed to help release
the chain from one position rather than to help lift
it onto that spot. This incorrect placement effectively ensured that the chain would default to the next cog rather than to land where the customer had expected. Neither the body nor the freewheel cogs were at all worn or broken, they were just not used where
they were designed.
Considering a transplant of a cog from one manufacturer's freewheel to another could
be possible... but, it would more likely be a long-shot. I hope you would have specific data confirming the perfect suitability before bothering to disassemble anything.
If not, this would be like assuming that you could grab any chainring off of any model of Stronglight crank and simply use it on any other model Stronglight crank... or, worst still, thinking that you might use it on any Shimano or Campagnolo crankset... simply because the number of teeth conveniently happens to be precisely the size you are looking for.
In spite of what I've just presented... I'm honestly NOT
trying to crush anyone's project.
But, I would warn that websites like Sheldon Brown's and Loose Screws which offer rebuilt customized freewheels tend to make anything at all seem possible, and make it seem supremely easy. - In reality, the work done for both of those websites is at the hands of people who have decades of experience. AND, they also have a huge stockpile of the "correct" and completely compatible new-old-stock freewheel cogs to choose from.
I have maybe 20 spare freewheels from several different manufacturers and I know for a fact that the cogs of only 3 would interchange at all with each other (all Reginas, and of the same model), and only then if the cogs were placed in the correct position on the body.
I had a Regina Oro freewheel built up to be the perfect original cog pattern for my Touring Paramount. This was a pure fortuitous fluke! I happened into an old local shop which had in their store room a box of maybe 300 loose Regina Oro cogs. Even among those, of the 2 specific sizes I needed for the re-fit, there were perhaps 40 of the needed tooth size... But, when tested for the correct thread orientation and "ramp" location on the freewheel body, only 5 worked.
I know this because as part of the "deal" I offered to help sort out what was a loose jumble of cogs left to gather dust many years earlier.
Sometimes diversity and lack of uniformity between brands and models truly sucks!
Much as I love old components, I'm absolutely thrilled that it is now finally
so VERY easy to find simple Shimano or Campy compatible freehub cassettes and individual cogs with simple spacers cogs.
Here is an example of the complexity of one freewheel model from Regina. Notice the different required locations for the various cogs to fit into the different diameter locations on the body. The threads could be either clockwise or counter-clockwise depending on their position in the array.
And these did not even use directionally profiled cogs!