Water quantities for big beer

I'm going to attempt my first "big" beer tomorrow - a Tripel that will come to 9.1%. I have a 3 gallon setup and will be using approximately 8.8 pounds of grain, with an additional 1lb of candi sugar to throw in the boil. The rest of the beer's specs are listed below:

OG: 1.089
FG: 1.019
Brewhouse efficiency: approx 70% (and improving)
Yeast: wyeast 3787

Hoping for about 4.5 gallons of water pre-boil, my initial calculations have me using approximately 3.6 gallons of strike water and another 1.9 gallons of sparge water.

Will I be sacrificing the big gravity with this amount of water? Should I use more water for a bigger pre-boil gravity? My pot can only hold five gallons, but I have another 2.5 gallon pot that can take the spill over if need be. If use two pots, can I just throw the hops in one and then dilute it with the other before cooling? Should I upgrade to a 90 min boil?

Thanks!

You've got a mash ratio of 1.6 qts/lb. That is fine for lower gravity beers, but you might want to scale that back a bit so you have some more sparge water.

For boil length, are you using a traditional pilsner base for your tripel? If so, then a 90 minute boil is in order.