Long story short, my '72 Charger (manual brakes) needs a master cylinder. It is the all steel one with 4 mounting holes. I've heard that there is one with a plastic reservoir that requires either little or no modification to bolt on to my car. Is this true? If so, what do I need to get one off of? Any insight is appreciated.

The plastic reservior master cylinders are also aluminum bodied, saving more weight. They use a 2 bolt mounting flange, but adapters are commonly available. An adjustable pushrod is used to make up for the thickness of the adapter.
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The adjustable pushrod isn't required but it doesn't hurt to have one. If you have a manual brake pushrod it should work just fine.