In the end I welded on some strengthening ribs which also serve as a flat bottom and stop the drain point getting damaged. Note that the old sump drain 'bulge' has already been flattened on the old sump so the gain is better than it looks here...
Stock pick-up pipe
shortened to suit the new sump. I also had to shorten the support piece. Normal clearance to the bottom is 16mm but I reduced this to both gain useful capacity and it was easier to just shorten the vertical section and braze it back together, I ended up with 9mm to the bottom. Apparently subaru rally cars are modified to only 6mm...
Finally fitted, the actual oil part of the sump is level with the bottom of the oil filter and I didn't want that to be the lowest point. All in I have gained about an inch ground clearance.