Cougar SV body on +8mm chassis. Help!

[Spoolio]

Ran my second hand SV Pro on Saturday and loved it. However, getting the shell to stay put was a 'mare and pushing the body pins in was an exercise in torture. Was running my Mini in the other rounds though so no time to really look at things. Now I've had some spare time I can see that the only way the shell will sit over the undertray properly is to have it located over the rear body post, but then the "waisted" section at the front won't sit over the front bulkhead/top plate as it sprags the sides out. If i move the shell forwards so it sits OK at the front then it wont sit over the undertray at the back. Do I need a different undertray for the +8mm chassis (although it looks OK to me) or is it just cack-handed design that means I can't use a standard shell plus an under tray on the longer chassis? What are my options, do I need a different style of shell? Really hope not as I got a new standard one on Saturday before I realised I had issues. Don't want to go back to the short chassis as I thought the long chassis was brilliant (even though my driving wasn't ).