I have a 74 23 Tsunami I/O, and did my transom from the inside.I know that many do the transom from the outside,but (just my opinion here)I feel the better way is from the inside.You don't compromise the hull ,this way and you don't have to spend hours laying up glass and fairing the outer skin back on.
Overall,it's more work doing it from the inside but I feel It's a stonger repair.
I started by removing the rub rail and the screws,to about midships,then I made a cut on the gunwhales in the center of the opening for the hawse pipes on each side.We could then lift the rear cap off.
Next we cut the inner panel that runs across the stern that has the lip for the engine box on it.
You now have an open access to the transom.I cut back the sringers,and removed the engine beds, and set a blade in the circular saw just deep enought to go thru the inner transom skin,and removed the peat moss core.
I laid up a couple layers of 1708 to the inside of the outer core,then put in the new transom core and followed up with a few more layers of 1708 on the face of the new core extending a good way onto the hull sides.
Another advanage doing it this way is that you already have the transom cutout in the "EXACT! location it was originally.
I used marine plywood for the core,glued together with Epoxy.I used Vinylester,and some polyester to do most of the layup from there on.
Here's a few pics if I can get them to load
