It kind of depends on the mobos power socket placement.
I have an Earthwatts 500 in a CM 690 (with bottom PSU as well) that is a bit bigger than the 300 and it is fine reaching the power of an Asus p5k-vm.
Looks like identical placement +/- 1 inch. The P5k-vm has ram at the top of the board and power connector right in line with the bottom of the ram. P5Q-e has ram shifted down ~1 inch from top of board and the power connector ~1.5 up from bottom of ram.
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