This may likely not be the input you are looking to hear - but here goes:
Pourable transoms aren't all that they claim. In an attempt to save time & work of splitting a boat open, you pay a lot of money for a very imperfect solution. Getting a good bond to the old fiberglass will be very difficult. Plus - there is no facing wall to hold the goop against the transom wall.
In old boats like the ones we work on, the best way to go is to split that boat open by removing the deck. It isn't nearly as bad as it sounds. Remove the old transom wood and if needed the floor. I think your boat may have a cored floor - no stringers. I can't recall. But building a new plywood transom will net the best results for the money. Correctly done, a plywood transom will easily out live you and perhaps your children. Nida products are nice but very expensive. An aluminum plate will cost a ton, but offer little to no added strength.
Keep in mind - transoms rot because water gets into the core. This is often a result of inadequate sealing of the original construction, holes getting drilled into the transom and not sealed correctly, and neglect. Most of these boat have sat out in the elements for over 50 years.
The key to making a strong boat is using the right materials with the correct methods of application. MDO exterior plywood is a cost effective yet extremely durable choice. You can use Epoxy or Polyester resin. The end results will likely be less expensive, take less time, and net stronger results. Dozens of people here on FG have done these (me, for one) and would be happy to help you through the process.
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