1080 ale in 70 qt tun?

Im trying to make a 10 gallon batch in my 70 qt. tun with SG of 1080. Working with grain/water ratio of 2qt/lb. Grainbill from BeerSmith is 30 lbs of 2-row. Mash at 149 for 60 mins or until iodine is negative. Problem is that BS (and my calcs) indicate I’ll be mashing in w/ 18 gallons of water at 157.

Question:
Are these calculations off?
Is it even possible in my 70 qt tun to get a beer with SG of 1080?

http://rackers.org/calcs.shtml/

can i mash it calculator says it will take 17.4 gallons with 30 lbs at 2qt/pd.

17 gallons is 68 qt. so your right there at the max. is it one of those coolers with a domed lid? some times they add that space in the advertised volume of the cooler. and your actual use-able mashing space is small than advertised. i would say you could do it even if you have to mash a little thicker.
ive been using a 52 qt cooler for a while need to get a bigger one i max out at around 1.065 for ten gal batches.

I think I’ll lower the ratio and increase my sparge volume. I can always boil a bit longer or (gasp!) augment with DME to hit my target.

BrauKaiser and others have mentioned mashing at 1 qt per pound without negatives from what I remember. Not sure how that would affect a bigger beer.

I have a 60qt MT and made a 1.086 Imp Stout-10 gallons, think I mashed @ 1.2g/lb, it was pretty full, but it fit and I still got 75% efficiency