Big beer brewing is a whole different animal as I found out. I do it about twice a year. The biggest thing I noticed was a drop in efficiency using so much grain. By the time you get your grist/water ratio right, you won't be able to sparge very much before you have your pre-boil volume. You can go on the thick side, but that introduces other considerations. I have resorted to mashing to about 1.060, sparging as usual (fly sparge is my default method), then making up the rest of the gravity points with DME. This keeps my mash process on track without too much fuss. There is no problem mashing all that grain, most folks do, you just have to anticipate some differences in your results.
The other thing I do is brew a standard gravity beer first about 2 weeks before the big beer. For an Imperial Stout, I'll brew maybe an Oatmeal Stout 2 weeks prior, rack the Oatmeal Stout on brew day, then put the Imperial right on top of the yeast cake. This assures a good quick start and saves you from having to make the gargantuan starter you will need for a big beer. And this sucker will ferment like mad. If you can keep the temps down, especially at first like someone else recommended, you can try to keep it contained, but for sure plan on a blow off tube and check on it about every couple of hours through the first 24 hours or so - that or have a really big blow off bucket.