3 helmets drying prior to sanding.
The visor is tough. This has big mohawk-like ends on it like the bucket, but you need to chop them off. Plus a whole bunch of the rest of it. Cut each side down to about 3.5" (I left a little more in case my angle was bad). Before applying any sort of glue, you'll want to do a couple of thigs:
1) Trim as much as you can on the sides of the bask and visor/bonnet near the places they intersect. I didn't take great in-progress pictures here, but basically anything that impacts how the pieces sit on each other and anything that would be difficult to access beyond assembly should be trimmed. (I left most of the front alone so I could trim it post-gluing to better match the sides).
2) Test fit and clamp before you touch any glue. You'll need to do this to refine how your visor is cut. Here you can make your cuts to better align the two sides, but make sure you look at the helmet from the side to check angle relative to the snout, and from the top to make sure it's not bowing in or out weird. Additionally, there are molded lines on the inside of the sides and the face mask that should align that can help you (I didn't get pics of those).
Pre-glue - with clamps on the side and the front face of the visor untrimmed. After my test fit, I trimmed the lower portion of this more, because I didn't want to risk damaging the face mask later.
Check your snout angle. It can slide around on you.
I did go back and trim the eyes before gluing, because I was afraid of hitting the visor when I did so.
Once you've glued it all, you'll need more Bondo to cover the seam.
After you do a good amount of sanding, you may need some gap filler to fill voids in the Bondo for the bucket and the visor. Thus, the red spots in the next pic...
Once everything is good, you'll need to prime it (the grey bucket above). This will highlight every freaking imperfection of what you just did. If there are still pits and edges, sand some more, apply gap filler, sand some more, and re-prime.