You'll need to take them out to fit the backplates especially on a sli setup.
You could technically do it, but I wouldn't recommend it.
Depending on the backplate/block, some reuse most of the screws that hold the block in position, I wouldn't want to risk getting the thermal paste being borked if you do it in situ.
Wait it out m8
GPU sag is a horrible thing
The backplate for EK blocks goes on after the block anyway.
You'll need to remove (IIRC) 4 screws around it and install the thermal pads.
The 4 screws are on the outer edges of the board so shouldn't affect the thermal paste.
See here for installation guide
It even includes: