OK, first, I am not sure what is supposed to be the front/back since you have airflow labeled on here. Second, what size are these radiators, what kinds, what size tubing, what fans, what CPUs...?
CPU blocks...run in series, not parallel.
Edit: I did a quick Google on your G5. It looks like a mess to work with, however, what components are you getting rid of and what is being replaced...what is going to be left stock? You could make this work, but its going to be a lot of DIY depending on the components. You'd almost be better off tossing most of the stock cooling system and seeing what you have left.