FWIW on my '70 Challenger the OE park brake held fine with stock rear drums. As I run an manual transmission it matters and is noticeable. Good parking brake needed!

After upgrade to a Dr Diff rear disc setup it also holds fine.

It sounds like you have the wheel stations and basic brake adjustment in good shape so I wonder what is wrong? Maybe try tightening the E brake cable adjustment until it causes drag then back it off a little to make sure the cables are as tight as possible.

For basic brake adjustment, move the star wheels until you have slight drag after fully applying the brakes a few times works for me.