A stock (4.5") '21 rim clears the chain by less than 0.75" so a 6" rim is going to hit it.
The 6" rim is also going to require at least a 180 tyre which once again isn't going to clear the chain.
Even a 170 on a 5" or a 5.5" rim is going to be very very close.

Any reason for going that big?
I can understand going to a 5" or even 5.5" rim on a racebike to allow the running of gp250 or Supermoto tyres but anything bigger isnt going to have suitable rubber available even if you can get it in there and "working".

I say "working" because its still going to make an absolute mess of the handling.
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