Both the GMC 228 and 302 engines in my garage connect the oil pressure gauge to the main oil gallery on the lower left side of the block about 5 inches from the rear.
That started in 1951
The place on the front of the block that the Tee threads into has a small hole at the bottom, restricting oil flow and therefore reducing pressure into the Tee.
That depends on the flow rate of the pump. If its 5 gpm the pressure drop will be minimal. I would say less than 0.1 psi.
The drawing you posted shows a 3 way Tee with the 3rd line ending in an upturn right behind the generator.
The drawing is an exploded view from a military CCKW350 parts book. When you move the tee fitting back to the block the tube will be at the back of the block not the generator. Here is a drawing from a 1939-1947 MPB 100-900 and the CCKW350 engine side view.
