Okay. I took pictures and measurements of Shimano and SRAM 8 speed cassettes and a Suntour 6 speed ultra spaced freewheel.
While my cassettes can come apart via a small hex screw from the rear, I kept the 6 largest cogs assembled. Both cassettes have two seperate smallest cogs and a lockring. The Suntour ultra 6 is a Winner body. The bottom flange of the freewheel body raises the largest freewheel cog off the counter by about the thickness of a credit card, so the cassette bodies are placed on top of credit cards.
L-R: Shimano cassette, Suntour freewheel, SRAM cassette
L-R: Suntour freewheel, SRAM cassette
L-R: Shimano cassette, Suntour freewheel
Measurements:
Suntour freewheel cogs are almost 2mm thick, while the cassette cogs are about 1.75mm.
The stack height is:
Shimano 26.5mm, SRAM 27mm and Suntour 27.5
So I think it is safe to say an eight speed cassette modified to 6 cogs would fit nicely on this 120mm spaced hub. I'd use an 8 speed chain. I could also take a 9 speed SRAM cassette apart to see how that works.