Axzevos :
laviniuc :
The Modified Card's Specifications
Length
320 mm
Height
120 mm (From Top Edge of PCIe Slot)
Depth
60 mm (2.5-Slot)
5 mm Back-Side (Through Back Plate)
Weight
978 g
taken from here:
http://www.tomshardware.com/reviews/r9-290-accelero-xtreme-290,3671-2.html
But will it work? You can see my specs
standard card is 3.556cm depth. + 3 cm clearing - if it's min 3 cm between them that should give you the needed min 6.5.
but it will be a very tight fit could cause the top card to overheat....