In Reply to: Step drill bit posted by Jack - Iowa on January 19, 2017 at 10:49:18:
The step drills I'm familiar with and keep on hand in my shop wouldn't do what you want as #1, those drills are made primarily for much thinner sheet metal and related materials; and #2, before the thickness of the 1 1/4" portion of the bit could drill through the plate (unless of course you could find such a bit with the last step being 1 1/4"), the next step would begin to drill through.
Perhaps a better quality, high priced and thicker step bit is made for the purpose you're seeking, but I've never known of such. If the hole you want in the plate has to be perfect in size, you may want to take the plate to a machine shop for the job. If the hole doesn't have to be perfect, most experienced welders can cut fairly precise holes with a cutting torch.