Man, that castle nut looks like it's about to fall off! Are you really gonna run it like that?
If it's for the cross-drilled disk that you've pushed everything out, grab some 993 rear disks and bolt them on instead. You don't need a spacer then.
You may find that you'll have too much of a rear brakes bias with the 951 front calipers on the rear. It depends on what you will be counter balancing them with up front.
I have fitted 951 fronts on the rear of my 951 but I've fitted 928S4 calipers up front and will be running a bias control.
The only calipers I can think of which would balance out the 951 fronts would be 993 TT front calipers.
Looks sweet though.
MG