I have worked on Edelbrock heads that had Hughes shims that were flat and not tapered, they made the support crack but they did not fall off or fail because the longer bolts still keep the parts together. These RPM heads were helicoiled which took even more off the goods making cracking very easy to happen... Check the stands by the exhausts, they are very skinny!! I filed the shims with taper, and bolted together, still works OK.
BTW, your solution now is similar to what is used in aircraft parts, I have a huge carburator from an R1820 Wright radial engine , and it has steel inserts in all threaded holes, to prevent pulling alu threads.
Hope you have cured the problem with the long bolts. Check that all your bolts or studs engage deep into the head!
Good luck!