There are some machining operations that require special fixtures. In this example, the plate is 18 x 23 x 1.25". There are threaded and reamed holes needed at all four edges of the plate.
A floor type drill press would be a good idea to use because it has a great vertical range. However, the holes locations were just too specific and precise. Processing the plate on a drill press would not provide accurate result since you can not position and then move the plate accurately.
The use of a milling machine is also a great idea if you have the necessary vertical distance between the table and the drill chuck. The table of a milling machine can be moved accurately on the X and Y axis (horizontally). Unfortunately the available height is a problem. Generally you may be able to process only up to 12" high objects. Conveniently the head of a knee type milling machine can be swung and slide, so you could easily place a larger plate beside and below the top surface of the table. This takes us closer to solve the problem and come up with the right solution.
For this application a drill fixture was built. The plate to be drilled can be clamped on the fixture to maintain a vertical position securely and safely. The plate is sitting on two horizontal support bars. These bars can be placed on the fixture at different heights at every 4 inches. The drill fixture has 4 casters, so it can freely roll with the clamped plate. For positioning, the fixture has an extension bar that is bolted to the fixture and then to the milling machine's table. This makes it possible to move the fixture accurately with the movement of the table, either on the X axis or on the Y axis. The fixture can be positioned and then mounted to the table to roll on the X axis, or placed and mounted 90 degrees, so it can roll on the Y axis.
Once the drill fixture and plate is positioned and mounted to the mill's table, you can find the datum the same way as you would find for any smaller part that was e.g. in a vice.
