The main bearing caps are a pretty tight fit__side to side__in the block; #s 1 & 4 in particular.
That fixture will extract them; #s 1, 2 & 3 with the 1/2 stud, and #4 with the pipe thread adapter on the stud (the internal thread is 1/2" SAE fine).
The same fixture (less the adapter, IIRC) also works on the MGB block (3 or 5 main).
Using a hammer to remove the main caps is expressly NOT recommended.