First- that's a really sweet bike. Whether the whole frame is Tange Mangalloy or the frame is CrMo and the fork and stays Mangalloy- That's a good quality frameset. One of my big regrets is selling off my Mangalloy Trek 420. Figure your CrMo/Mangalloy frameset is about equal to a contemporary Trek 600 level bike. That's a good bike.
Second- wow- that thing is in great shape. Congratulations!
Third- what is you gearing on there? It seems to me you're kind of confusing freewheels, cassettes, axle length/spacing, number of cogs and cog size.
IMO- the number of speeds (rear cogs) only has importance with the steps between gears.
Generally- the more cogs, the wider the spacing between the dropouts - for freewheels- that means a longer axle. A 6 speed freewheel has a good distance for the axle to not be overextended.
The size of your rear cogs has no bearing on your axle length.
The size of the rear cog is limited by the ability of the rear derailleur to shift onto and off the rear cog- which also takes into account the length of the dropout hanger thingy. Generally, an ATB rear derailleur will have a greater range- it's a matter of the design of the derailleur, not the cage length. (a lot of people think a long cage derailleur will allow you to use a larger cog- the longer cage takes up more chain for a triple crankset, which generally come on bikes with a larger range of gears).
IMO- I like low gearing- although no one should tell you what you *should* have on your bike- IMO- a 28 or 30 big rear cog with a 40 or 42 chainring should put you in decent 'road' hill climbing business. It looks like the Shimano Megarange 6 speed freewheel gives you 14-24 and then a 34 bailout. (and magic marker over the billboard Shimano branding works well
) I don't think your Golden Arrow RD will work on a 34 tooth cog.
As far as what you want to do with the bike... It's not a super collectible bike, so you can mess with it as you see fit; it's a good quality frame, so you can "upgrade" parts and stuff- you can end up with a REALLY nice bike.
Congratulations and good luck!!!