Based on your latest batch of picture, here is a new picture found after further web searching.
I clued in on the first of your pics which shows a different side mount bolt pattern than the one in my previous picture.
This new picture displays the same side mounting bosses as yours, but shows this particular year group of block (50-54) has the casting number forward of the fuel pump mount point (down close to the oil pan rail area in right side of photo).
Thinking the elusive casting number is just above the oil pan flange in your last pic, and you may need an angled inspection mirror to see into that spot.
Sorry this is turning out so difficult, but hoping this will help finally locate the rascal.
