When it was built, the wood of the stringer touched the transom wood and then they glassed it. Then drilled a hole in each stringer to allow drainage across stringer to the center bilge. They did not glass the holes that they drilled so the wood of the stringer rots at the bottom of the transom.
I did not fill the stringer. The pour leaked through the transom & out the hole in the stringer. I just jammed towels and rubber gloves in the holes as it leaked and it cured that way, then I re-drilled the holes in the stringers.
At minimum you should lightly glass over the holes in the stringers before you pour. And ALL holes in transom, inside and out. And anywhere in the interior transom edges that you suspect it might leak.(since you are not reglassing the inner skin, I would suggest one layer of glass along the inside corners as insurance against leaks)
Mix 2 full buckets. Without leaks, 2 buckets of carbon core will fill to the 25" cutout level. Let that harden then glass over the transom and sides. Mix third bucket to fill up both sides.
A thought...even if you are going to use a 20", Build up a dam and pour to the 25" level straight across and later cut out a notch for a 20".
If you are going to use 25", you need to build up some serious skins
As per video, remember that you have to keep the shape of the transom so it doesn't bulge out and harden. especially, make sure the motor mounting area is straight and flat.
Remember that the transom on a V20 is not flat. Look at it. The motor area is flat and the sides angle forward.. It was actually 3 sections of plywood.
It might be tough with the full inner skin in place but I would glass over the holes from the inside. You will drill them out when cured
USE POLYESTER RESIN!
If you have any ideas about using any extra carbon core, have the area ready to pour after filling first and second pours. For instance if you have a small area of deck rotted near a screw hole, cut the top skin, dig out rot, patch bottom, pour extra in there and later glass over top....but you have to be ready because it sets.
Wear gloves. Put cardboard on your driveway.
This scraper from Home Depot worked great for scraping wood from glass