I would highly recommend a parti-gyle like what Mort did. Plan on 50% efficiency and use only the first runnings. Then you can batch sparge again for a 2nd runnings beer. If you have 2 boil kettles you can have the 2nd runnings boiling within 15 minutes of the first runnings, or just wait until the RIS is done. Kai has good info on his website: http://braukaiser.com/wiki/index.php?title=Batch_Sparge_and_Party_Gyle_Simulator
One problem is the mash will be bigger. How big is your mashtun (or tuns)?
Or just add some DME to get up to the gravity you want.
And if you haven't fermented a big beer like this, you will need lots of yeast (I like to use a 5 gallon "starter"), lots of oxygen (you can re-oxygenate up to 24 hours later), and lots of temperature control. Lots of yeast with lots of sugar like to make lots of heat.