65 Barracuda w/500ci RB, Stage VI Max Wedge Port Heads, 6-speed, and 5.38 Dana
I remeasured with my precise caliper and discovered the dowel pins are 0.371". I called my best friend and off the top of his head he says, "That's the same size as big block valve shafts." Of course, I don't have any used ones laying around as I got rid of all of my spare big block stock heads and assorted parts, but fortunately he had one that he sent me. I cut the valve lock area off of it and cut it in half. There was just enough left for going all the way through the adapter plate and the bellhousing.
As you can see in the last picture, the input shaft extends past the front of the block protector, which means it will engage well into the pilot bearing in the back of the crank, once installed.