You have already touched on this when you questioned the use of a small sparge volume, but that big a grain bill is going to probably lower your efficiency by a significant amount. With large grain bills and a fixed volume of wort and fixed boil time (say 5 gallons after 90 mins), the proportion of strike water to sparge water changes. Or, in other words, the amount of sparge water relative to strike water decreases as the grain bill increases.
There are two common ways to deal with this:
Sparge more and collect a larger volume of runnings, necessitating a longer boil to get down to 5 gals (say 2.5 hours instead of 1.5). But this may generate problems with 'caramelization', which could be an issue for some styles.
Accept that you will lose efficiency, and increase your grain bill to compensate. Then you can keep your wort volume and boil time the same as smaller gravity batches. The difficulty here is knowing how much efficiency you will lose. On my system, the difference in efficiency between a 1.055 batch and a 1.090 batch is about 8 - 10%, but I am sure that varies widely among breweries.
Hope that brew goes well! (I just did a SMALL grain bill batch -- an English mild ale -- and I had some opposite problems and I am still kicking myself for how I handled it all! Live and learn!).