I would use a later dual drum M/C, like from a 68 Fury. It will bolt in place of you existing M/C.

What are "HD brakes"? that is unclear.

Do you mean power brakes?

You will need to mimic the all 68 drum setup for the M/C connections.

Front port connects to the rear drums. Rear port will T to the front brakes.

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They say there is always the exception that proves the rule.
Don't be the exception.