This link will tell you what you want to know:
At 1.25 Q/LB you can use the 8 gallon pot.
I wonder if your mashing will be less efficient at 1.0 Qt/LB compared to 1.25? Will it be harder to gelatinize the sugars? (I read that in a paper on HBT somewhere). Or maybe its a positive , as the enzymes are concentrated.
I do BIAB, but since my stove burners are weak, I use two 5 gallon pots.
I am soon going to brew a Bourbon Barrel Old Ale with a 16.5 lb grain bill.