My bad I wasn't clear.I get what you're saying, but I still can't find any source for 44mm - and how do you know the backplate is 4.2mm or that the metal plate is 40mm?
According to Dan's website, the A4 fits gfx card
Without backplate: 40 mm
Including backplate: 45 mm
Think of the pci-e slot as the "starting point"
So 40mm of space from the pcb to the bottom of the card (relevant b/c pci-e slot can't move),
and 5mm of space from the pcb to the top of the backplate (irrelevant),
The XC3's all protrude 4.5mm from the bottom, so you probably need standoffs in the A4.