With such a huge beer you definitely want to do a two stage fermentation the way you outline.
The way I do it is first off, mash at 148-147 for about 90 minutes and do a slow sparge. This will get the most fermentable wort possible.
Next, I do wait until the fermentation just starts to slow, somewhere around 72 hours after first visible signs, and add the sugar in a thick solution (maybe 1 quart of water.)
With this much maltose you really want to keep the simple sugars on hold until the yeast have worked their way through the more complex ones.
Also this yeast works best in the mid 70's.
On Tap: 1. Kelly R. IPA, 2. Roter Hund Hefeweizen, 3. Bud Killer Blonde, 4. Red Dog Pale, 5. Roter Hund Oktoberfest, 6. Pumpkin Ale, 7. McRed's Stout (with new nitro system and stout tap,) Cream Soda, 8. ESB # 3, & 9. Ordinary Bitter.