Hi Mike, it is still work in progress. I think the main frame is strong enough, the problem is where the selector housing is mounted to the frame. There is a bit of movement there.
The original casting of the extension is all aluminium as well.
I am going to look to see if I can knock up something to mount it to the tunnel as others have mentioned. I have googled cast aluminium welding and see some use the HTS 2000 brazing rods, anyone had success with this? I would be able to braze mounting brackets onto the selector housing and mount it with bobbin mounts.