Here's an update, Using angle grinder and cutoff wheel I removed a little over half of the 'ears' on new headlight bucket to help improve fit. Well, area in front of 'ears' now was rubbing on casting, so I ground off a bevel on front side of casting raised mounting area, which now clears the bucket (and also helps to guide bucket onto mount when installing).
Couple observations: the new headlight is both smaller in diameter and is recessed towards rear compared to original headlight. I mean it looks good, except in my case it doesn't line up with dart windscreen as well as original light. Not a deal killer, just definitively not as trim.
orig light diameter: 7 3/4", new light diameter: 7 1/4"
orig dist from center mount to front of bezel: 2 1/2", new dist: 1 3/4"
Just one more observation, I now have make sure and really tighten lamp swivel mount bolts to assure that it doesn't move. Orig light had a larger contact area with casting, new light is roughly half that and it takes more friction to assure it won't move. Probably is also due to the butchered 'ears' not mating as well with casting.
One more thing, the paint on Motodemic bucket is cheap. Definitely not powdercoat. Scuffs easily. Used a marker to touch up satin paint on new bucket and GRiSO casting. Actually doesn't look too bad.
Ok, last thing, the 5mm mounting bolts don't engage as many threads on new bucket. There's a gap at outside of ears where bolt has to enter in a ways before it contacts threads. Orig had threads all the way to the outside. Now, with windscreen mounting bracket, there's maybe only 3 or 4 turns total, (not sure, didn't verify that). Took off outside washer and used loctite on threads. Seems good to go.