I had the same boat up until a couple years ago. Mine was a 1986 with a 3.0l I/O mercruiser (great engine). Mine came with the 9.9 merc 4 stroke kicker, plumbed into the main tank just like you are looking to do.
Just a little tip: The batteries (x2) were located at the back of the boat, port side (the trim motor was located on the other side). When the previous owner added the kicker in 1997 they also mounted it to the port side of the transom. Between the 80 - 100lbs. of batteries, and the 150lbs. of kicker motor and mount, the boat had a serious lean to that back corner. You really had to move gear and people around in the boat to get it to run level sometimes. Also made it very difficult getting it on the trailer level. I had Greg at North Erie move the motor to the other side and it balanced it right out nicely.
Just something to keep in mind when you're adding the kicker.
Other than that it was a great boat. These are built for the Great Lakes. I was caught coming in against some 5 footers a couple times and this boat did great. Enjoy!