I've got a project that requires a 1" square solid spacer to locate the brake caliper further out from the mount. I recall an issue with using aluminum between 2 cast iron parts due too heat expansion or something.
The spindle and the brake caliper bracket are cast. Attached via longer shouldered bolts. Painted to prevent corrosion.
Machined 6061 would actually be cheaper & lighter. I've seen AL used as brackets but sandwiched feels like a no no.