Installing WP guage???

I'm in the process of hooking a water pressure guage up to my 1990 Johnson 200 GT and I'm lost. The man at the dealership told me to hook it up to the left side of the block or if their is already a tap and hose there then T into that hose. Well my problem is there is already a hose T'd off the hose going into the pee outlet and it runs back into the block(I'm assuming it goes back into the block, I'm not mechanically inclined when it comes to motors). Where would I hook this thing into it it already has a T on it?

The hose that you are looking at goes to the power boost cooling valve. Look at the adapter plate there should be an allen head bolt that can be removed if not tap in below the inverted T.