I have an 08 RoadGlide, looking to upgade front to 19 inch but i have the 16 X 3 rear wheel. Wondering if its possible to machine the rear wheel to match the front. The machined/blacked area that comes into the lip of the rim under the spoke is what i am referring to.

Front

Rear

 

Sure it is ... you can do it by hand (ie. Dremel or die grinder)

OR ... you may just want to do it with "Paint" ... creating the illusion of the recesses

 

I wouldn't bother personally - looks just fine to me! They are both 5-spoke and black/silver.....