I think I can probably answer this one for you! I'm battling with the exact same problems.
I would strongly recommend you extend your backbone of the network down to the transom area, and then have a double red 'T' and a terminating resistor on the end. There is a limit on the max length of the individual device leads to the where they plug into the network, so it's best to have the network backbone running the length of the boat and the device leads short.
You will then need an LGC 3000 GPS aerial (which is the red connector one) to connect onto one of the open plugs on the new tee. Depending upon whether your EP-20 (990C0-88011) is a blue or red (I suspect it will be blue if ordered form Brownspoint), you may need a blue to red adaptor to conenct the EP-20 onto the other spare tee. If you do need this adaptor it is a lowrance MRD2MBL. Good luck
FWIW, the new red Lowrance connectors are the same as the industry standard Micro C.
The converter 990C0-88035 is for adpating the standard EP-20 to connect to the 2008 and newer outboards. You won't need this.