There's a disc conversion kit out there, somewhere, I've seen for the GMT400s that overcomes one shortcoming of the Little Shop kit (and other kits like it), on first glance anyway.
It uses a more recent caliper bracket and caliper assy, akin to this one pictured, with abutments (which Little Shop doesn't have) which properly transfer the braking forces back to the mounting bracket.
The kit I saw didn't have a parking brake though. Maybe there's another, somewhere, that does.
And unless the mounting bracket's quite rigid, the caliper will tend to twist during braking.
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