The holes measure 13/4 inches straight across. The plumbing measures 1 1/4 inch for the drains and 1" for the return also measured with a tape measure straight across.
Somehow I am overthinking this. The holes are 1.75 inch but the return line pipe that came with the tank is only 1 inch and the stand pipes are 1.25 inch pipes. How can 2 different sized pipes fit in the same sized bulkheads?