wasn't me...I replaced the hood as well.
But to give you an idea of the "tapering"...
...the newer hoods overlap the inside upper corner of the lights so the lense is a bit recessed there. If you look at the picture above, the hood actually goes around the grill. The part of the hood on the sides of the grill is what overlaps and thus the lenses are recessed in that portion to accomodate the protruding sheetmetal. In the older style hoods, the grill attaches to the portion of the hood where it ends, and doesn't overlap.
The net effect is if you only replace the headlamps, the portion of the lenses which the hood is supposed to cover would show.
Someone recently crashed a 400E. If you go look up the thread and follow his link, you'll see a picture of a newer headlight assembly on an older style hood.