From my limited experience, and believe me it is limited, you won't fully know what is going on there until you take the skin off the outside - or inside as some prefer to do. On my '73 I had the tranny re-bonded (good wood) by a pro from the outside. He ground the skin off about 2" from the edges all the way around, then glued and screwed it back on then re-glassed and faired out the skin all the way around the cut. Previous owner had done a half azzed job with new wood but did not bond (aka "glue & screw") skin back on. Total job cost me $1200. In hindsight, with a little knowledge and the willingness to sand and fair I could have done it for a couple hundred bucks.....but I sold the boat after that and bought a V21 so it didn't matter.
The gaps were pretty major around the edges. He had to use a lot of filler to fill it in. This is something I would not have known about unless I drilled holes all around. I wouldn't recommend drilling that many holes just to test the wood. Just my opinion. There are lots of other glass guys on here that can advise you better than me though...stay tuned.
Here's some pix....
Outward appearance was a decent transome job however a crack developed soon after I installed the jack plate:
This is the transome after it was professionally replaced and it was as good as new:
