Bruce's rear axle tube is not a Camaro piece. Looks to be Pontiac but I'm not sure if it's Firebird or borrowed from something else.
Below are the JL8 bracket and drum brake backing shield. On the JL8 bracket, the distance between the upper pair of holes is 3-3/8" or approximately 86mm.
The bottom pair of holes on the JL8 bracket is the same distance across (3 3/8").
On the Camaro drum brake backing plate, the distance between the upper pair of holes is 3-3/32" or approximately 78.5mm. The lower pair of holes are
closer together and measure 2-21/32" or approximately 67.5mm.
I'm going to take a guess and say Bruce's flange hole pattern is neither one of those and that might be why they had to fabricate their own caliper bracket.
Bruce, if you ever get a chance to measure that bolt pattern, let us know what you find out.