Which then begets the next question: What size HRC rear best goes with a 3.5" front? Is there a readily available rear wheel similar to the VFR400 front that would make a nice match?

Here in the states getting race rubber in 110/70-17 and 150/60-17 can be a challenge. And forget about the MC18's 18" rear. They have to be shipped in, I believe from the UK. I have been running Bridgestone BT-090 and BT-003 Race/Street and they do indeed wear like "iron" but am looking to try out something more sticky and perhaps slicks.

A 3.5" rim allows for the much more readily available 120/70-17.
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MC21R9N track
MC18R2J track
CBR954
CR250