Tandem Cycling - six-speed freewheel?

Helping a friend with a early 90s tandem and it needs a new freewheel--cogs warn and chain skipping madly. The existing Shimano 600 (MF-6208) 12-30 freewheel is narrower than the current six-speed freewheels being produced by Shimano. Also, the LBS questioned whether these inexpensive freewheels produced now are stout enough for tandem climbing.

So, are there any recommendations for a replacement freewheel, with at least a 28-tooth cog, that would fit? Or should we just look for some NOS on Ebay?

If the internals of the FW are still good look for some replacement cogs. Bike shops used to keep them around sometimes on a pegboard display in the mechanics work area.
Went Byebye when cassettes were born.

Was told that IRD's (http://www.interlocracing.com/freewheels_steel.html) freewheels do not have the lock nut design that renders the current Shimano six-speeds a bit wider than when originally introduced. IRD also offers a 32-tooth cog making it all the more attractive. So have placed an order for one and will find out. If still too wide will cold-set the rear triangle. Also ordered a five-speed for an even older Santana.

It's not the lockring that makes the freewheel wider, it's the overall spacing. The MF-6208 Uniglide freewheel had 5.3mm c-t-c spacing while the current Hyperglide freewheels use 5.75mm spacing.

It's not the lockring that makes the freewheel wider, it's the overall spacing. The MF-6208 Uniglide freewheel had 5.3mm c-t-c spacing while the current Hyperglide freewheels use 5.75mm spacing.

That's useful info. Does that mean the indexing will be off? Will be interesting to see what the IRD is.

Just to conclude the IRD six-speed fit without issue and performed well on its initial ride.