Another 6-speed Uniglide user here...
I can see splines where the small sprocket goes on, so coupled with the fact that the freewheel unit had a 10mm Allen key fitting I'm happy that it's a Uniglide one and not a Hyperglide. As you probably know, the Hyperglide has a splined tool that you need to remove it and on the Uniglide the small sprocket screws on securing the cassette.
It's quite a big diameter thread and it could have been on there for quite some time so it doesn't surprise me that it won't budge. Persevere with WD40 or Plusgas. Mine has been off a couple of times, but I still need to stick the bike in a high gear with the chain on, foot pressed firmly on a pedal and give the chain whip a damn good belt. You're definitely undoing it the right direction?
Conversions: I'm fairly certain that the 6-speed is narrower than the 7-speed. The spacing between the gears is definitely different because the spacers are a different thickness on the 6- and 7-speed Uniglides, so if you have indexed gears it won't be compatible. Pretty sure the 7-speed Hyperglide is the same width as a 7-speed Uniglide and the spacings are the same for indexing. I've heard mixed reports, but I think you can swap 7-speed Uniglide cogs onto a 6-speed if you use the 6-speed spacers.
It's the mixed reports about this that have led me to nurse my 6-speed Uniglide. This stuff isn't cheap any more and it's quite a lot to splash out just to find out if it actually fits. I'd say if the bearings are still ok then do your best to get that one apart and maintain it regularly to keep it reliable. The sealed freewheel inside the hub can be problematic, but just soak it in petrol, thinners or WD40 to loosen the dried out debris inside, dry it out thoroughly and then soak it in oil to re-lubricate it. You can dismantle them but mine wasn't keen to budge when I tried and I didn't want to risk breaking it.