Sills all off

After a couple of days of grinding I have now finished removing the sills completely from the Off side and I am now struck by the fact that there was no centre strengthening sill? This also leaves me with the quandry of how on earth do you weld them in.

The inner sill has a lip that slots over the top hat sections and the centre has a lip that points towards the inner but they but up together at the bottom. My only thought at the moment is that I should weld in the inner and then try and weld the centre along the outer radius of the bend for the top lip... any suggestions?

In the above image you can see them both roughly offered up and maybe you can see what I mean (the silve one is the centre and you can just see the black inner on the right).