For such a big beer, you could:
- Mash thicker, to get a more balanced mash and sparge runoff which increases efficiency
- Boil longer, so you can sparge more, collect more pre-boil, and get a higher efficiency
- Do a second runnings beer, to avoid wasting all the leftover sugar. You should prepare for lower efficiency for your first beer when doing no-sparge... see How to Brew for more details.