I do this every summer, but I do the AG version. I did do the extract version twice back in 2014 according to my notes before I converted to AG. I would NOT recommend trying to boil 6.5 gallons in an 8 gallon kettle. The recipe is formulated for a 6 gallon batch due to the amount of hops that will absorb extra beer. When I do a full boil for this, it nearly maxes out my 10 gallon kettle and can be touchy for the hot break phase to not get a boil over. It's a 90 minute boil for both versions (AG and extract) so you can figure roughly 1.5 gallons loss due to evaporation, depending on how hard you boil. I would personally say that if you were trying to bump up the boil volume, I would go for like 5 - 5.5 gallons max in an 8 gal kettle.
I can personally account for the massive loss on this recipe due to absorption, so I would say you want to get as close to 6 gallons final volume as you can. I have a 6.5 gallon carboy I use for this and it just barely holds it with a blowoff tube. As others have mentioned, you want to get close to 1.070 so you can tinker with volume based on that.
The 100 degrees will come down when you mix with the 2 gallons of cold water for your top off volume as @jmck said, it's a cooling trick. I chill this down to around 68 before pitching yeast, in the summer my chiller can't get that low so I put the carboy in a big plastic bucket and rotate out ice bottles to bring it the rest of the way down.
Also, for your hop stand, I'd recommend give it a nice manual whirlpool with your stirring spoon, then cover it and let it sit. I personally use a pump for hopstands, but it's not necessary starting out. The easiest way to sanitize your chiller is to put it in the kettle for last 15 mins of boil, so I use aluminum foil to cover the kettle around the tubes as my lid won't fit on with the immersion chiller in.
I love this beer and brew it every year side by side with Off the Topper. Feel free to ask any questions and I'll be more than happy to help as best as I can.