I'm also not seeing why that would be needed. Are you just trying to add the GPU into your already existent CPU loop? You would just take a line going from the output of the CPU block to the input of the GPU block. Then outlet of the GPU block to the same spot the current output of the CPU block goes.
With your system, lines go like this:
- Outlet of the pump/res/rad combo goes to inlet of the CPU block.
- Outlet of the CPU block goes to inlet of the GPU block.
- Outlet of the GPU block goes back to the pump/res/rad combo.
No need for any special fittings, and the more you add in the more restriction you add and the greater chance of leak you get.
Keep it simple, and use the least amount of hose and fittings possible.