This ^^^get a hydroboost and matching master cylinder from a 2500/3500 truck (use a 1994 k3500 diesel or 454) ... You will have to modify the brake pedal or find one for a hydroboost at a wrecking yard. I would just find a complete system from the wrecking yard.
As noted, the pedals are different for the hydroboost brakes, so consider taking them from a donor.
Too, the hydroboost systems have a tube-and-fin power steering oil cooler mounted in front of the radiator. Grab it, too, otherwise they're easy to find on eBay.
(Edit: Some but not all have the tube and fin cooler.)
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