I have no idea. I don't do restoration work so I have no experience in this area.
I would however pull the trigger and buy the replacement aftermarket bumpers iof the restoration house says they are an exact copy (dimensionally, not quality, or originality wise).
As to the holes I doubty it as that was an optional piece o even the factory bumpers were probably drilled on the line to install them. If so use a new drill bit, oil to lube the bit and a half inch drill motor set for as slow a rotation as you can and apply a lot of pressure.
Most people with after market bumpers complain about the quality (poor chrome plating process) and not whether they fit or not. You can see the black color of the chrome metal in most aftermarket bumpers and not the reflective depth of the chrome of an American car of the era. This is because chrome is much more expensive now (why the factory used polished stainless steel now instead of chrome plated mild steel).
To do it properly the steel is smoothed; with all dings and scrapes welded and ground smooth. Then polished to a glass flat surface. Then copper plated and polished to obtain a mirror finish. Then triple nickel plated and polished between each of the three coats of nickel. Finally one to five coats of chrome metal are plated on with polishing between coats. That is a lot of labor and three currently expensive metals just to get bling. Which is why it costs so much.
The process is also highly toxic to the workers in the plant, and the environment if there were spill. Because of this the process is tightly controlled by the EPA.