I did the transom on my Tsunami from the inside.A lot more work,but imo it's a better way structurely,as you are not compromising the hull by cutting out the back.
After I had the core removed to the inner skin,I ground the surface with 36 grit on the disc sander and then applied one layer of 1708 with no-wax polyester resin to the face of the skin.
The next day I applied a coat of polyester hull & deck putty(mix of resin,milled fiber,cabosil) with a notched trowel to the face of the skin,and also to the face of the transom.
We then placed the transom in against the skin and clamped and temporarily thru bolted it until we had a good even pressure pulling it all tight together.
We did this in the winter with temps around 35 degrees,which actually worked out great,as we had loads of time to work with it before the mixture started to kick.Once it was done we buttoned up the enclousure and brought the heat back up to 70 deg.,and let it cure.
We clamped the the top edge with deep c-clamp,and so as not to affect the outer skin,we used the holes where the transom eyes were,and the outdrive hole was to put the bolts through.We only had to drill two other holes.
For even support to clamp the outer skin tight to the new core I used 2"x4" heavy wall Aluminum channel that I borrowed from work.(scaffold handrail)
The hull & deck putty I used was from Merton fiberglass supply.It's a nice putty to work with,and is very strong.This worked very good for me anyway.
http://www.mertons.com/
