I doubt it as the exhaust is different on the two bikes. The 390R has it up higher, whereas the Cup has it lower which presumably gets in the way of the longer belly pan.
My name is JB, and I'm new to the forum. I'm under the same moniker (jb.junior) on the Triumph675.net forum and have a lot of "cred" on there. I race in the CMRA and am located in Houston, Texas.
I raced D675's for the past few years and won a novice HWT title. After a bad wreck in the 2017...