Hi Daz,
I always when mounting a buggy shell start at the rear.
With all XX shells, the body is mounted from the rear. This is because of the rear tower/shocks and all xx shells are tight around the rear of the shock tower.
So fit the body at the rear first.
Then drop the front down. You can do 2 things.
1. Black grease on top of the mount, place the body on the mount and then you will have a black dot and a centre point.....
2. Drop the body on to the mount, then firmly push the body on to the mount, then you have a dent in the shell as a centre point.......
Hope that helps.........