Hello!
I don't know your specific card model, i don't have one like that, so I can't comment, because every case is unique and specific.
but I can show you what I did with one of my 7850's that died the primary fan (it has two, but only one report speed to the card)
this is the card (with the fan taken off):
so I prepare two pieces of aluminium 3mm thick i had around, drill and thread the holes, and screw it to the heatsink:
Put two Arctic F8 PWM controlled coolers on it:
If I knew how efficient F8 are, I will only put one, because 7850 doesn't need that much cooling capacity, but 7950 will need two for sure!.
here it is the card working:
overclocked at 950 MHZ in scrypt, fan stays at 20% speed (about 900 RPM) with the card at 55 degrees.
unbeliveable efficient.
if I had to do this on a 7950 I will go with the F9 ones that are a bit bigger!, and cost the same.
just remember, that only one of the fans connects the yellow wire (speed sensing, it is the separated one), all the rest goes in parallel, also, colors from the card are different, if you have doubts about how to connect the fans, post pictures of both the fan connector (the new one) and of the original fan and the connection in the PCB so I can help