While your boil off amount is correct, you are way off on the mash volume. No way an 8 gal pot fits that volume.
generally, for my 5 gal batches I use 7.75 gal of water and after I mash in I come to around 9 - 9.5 gal of mash volume depending on the grain volume. So honestly an 8 gal pot will not work for a full volume BIAB. However, that does not mean you can't BIAB.
What you will want to do is mash it with much less water. I would aim for a thick mash like used in a "standard" mash tun set up. 1.5 qt of water per pound. is the normal ration I believe. This should leave you with plenty of space in your pot. When you are done mashing, you can either dunk sparge the bag in the remaining water. (some people feel even cold water works for this) or you can top off the wort with the remaining water as you bring the wort up to boil.
Both methods limit the impact on your hops utilization in the boil. If I was limited I would probably go dunk sparge. Just my $0.02.