1987 R20 - rear bumper bracket info needed

[Southcity]

Friend owns a 1987 R20 with rear chrome step bumper which recently suffered a rear end collision. The existing bumper was mounted with outer brackets only (no idea why). Body shop has new bumper and brackets in hand (comes with inner brackets only) but has said that while brackets mate up fine with bumper, when attempting to mate up to frame rails, only two bracket holes line up to existing holes in frame rails, third hole would require drilling into frame.

Is it possible they have incorrect brackets?
Perhaps the aftermarket bumpers come with universal inner brackets only and some drilling is required?
Any info/guidance is appreciated.