Actually there is a small visible difference in the frames at the bottom front, but I don't believe it would affect mounting the engine. It might affect mounting the cooling system however.
Just looking at the parts drawings and comparing the 2016 FLHXSE to the 2017/18 version, the only obvious difference in mounts and links involve how the mounts are attached to the engine,and how the stabilizer link is attached to the head. On the TC there is a separate part that bolts to the bottom front of the cases for the mounts. On the M-8 the cases were changed so the mounts attach directly without the extra part. As for the stabilizer link, the link itself is the same part number, the only difference is in the brackets that attach to the heads.
There may be other less obvious changes, such as the durometer of the mounts to suit the different vibration levels for instance. One thing that needs to be researched is how the heads and rocker covers fit to the frame backbone. I can't tell if that area of the frame was changed (haven't seen any details for the 2017/18 changes like we used to see from Harley every model year in the past).
Take all of this with a big grain of salt, and don't order an M-8 for your bike based on my analysis. All I'm really saying is that assuming the frame backbone stayed the same, it should be possible to mount the engine to the frame. There may be other hidden reasons why you can't easily do it.
JMHO - Jerry