slot 1------ slot 2------ slot 3 ----- slot 4----- slot 5 ----- slot 6
1080ti ________empty______empty _____1080ti ____empty ____ mini 1080
I believe this will allow full air for the longer 1080ti
If it doesn't work, try one of the Gigabyte "ITX" 1070 cards in slot 6 - they're a good bit shorter than the Zotac, much less anything else.
https://www.newegg.com/Product/Product.aspx?Item=N82E16814125893
I end up with cards at 2-5-7 on my 3-slot motherboards, so same effective spacing as you're using - and no cooling issues for the middle card as it has one fan completely unblocked even on the 2-fan Gigabyte or EVGA cards I have.
If your cousin was a DT on Ranger in the '79-'80 timeframe, I probably knew him by sight if not by name, as we would both have been in OE division.
I sent a Pm