I don't think there are any actual cut outs. The tube that goes to the body has a filled end that is threaded and a bolt from inside the boot goes into that. There is a rubber grommet for the hole and I'd guess there is a load spreader washer inside. An extra brace inside to the chassis floor or inner wheel arches would make the bar much stronger but I don't think that was standard. The fronts are a bit beefier as they angle down to the chassis horns. You might have to slot the bodywork for those. Some reference pics here. I'm sure there are others if you search around.
https://classiccars.com/listings/vie...illinois-60073