One thing that may be doable is to leave the last 3 cogs as they are and only reduce the spacing on the first 4 cogs. You will be a little off on the lowest cog, but the rest will probably be fine.

However, breaking open a current freewheel to remove cogs may not be that easy. Older freewheels just required unscrewing the small cog. The new ones have lock rings. I once made a tool to work with the Sunrace lock ring, but the new ones are on too tight. I haven't tried to remove a current Shimano lock ring.

I still run freewheels on a couple of bikes and as much as I'd love to try it, it is even too much time and effort for me. So I run down tube shifters and 7 speed triggers and call it a day.

I will one day re-space a Dura Ace 7700 cassette hub to fit my 126mm Cannondale dropouts. I already have the wheels waiting for me to get around to it.

John