Hi!
I did have the same problem, had a 403 shaker on a 400 and that did not work..
First i drilled away the rivets so the plastic goes of, then i cut of that much it needs to come down enough and welded it togehter again. (metal plate)
When this is done i put the metal part of shaker on the air cleaner, after this is done i layed the plastic top loose on the air cleaner an carefuly shut the hood.
Now it is easy to fix the shaker in the hole and mark for new hole for the rivets. drill and mount.
(I compared to a stock 400 shaker)
The only differents you can spot is the non stock rivets.
Sorry for bad english..