To many assumptions here for decisions concerning brake systems.
For starters you need to measure your current fluid pressure at the caliper to determine if there is a problem with your master cylinder. You could have pitting in the bore that is bypassing fluid and preventing it from generating full pressure.
https://www.summitracing.com/parts/s...-hSQLBvuKgG4eE
Then take 15 minutes to remove the master cylinder and measure the master cylinder bore.
Then measure the brake pedal ratio. There are different ratios for manual and power brake cars.
https://shop.wilwood.com/blogs/news/...w-to-find-them
With this information you can run the numbers and know what pressure to expect at the caliper.
To go to manual brakes you will need to have the correct pedal ratio.
You haven't mentioned if you using the original distribution block or has it been replaced with a later model disc brake proportioning valve? Either one will work but if it ìs the original drum brake distribution valve that will cause the rear brakes lock up on hard stops unless you put an adjustable proportioning valve in the rear brake circuit.
https://www.jegs.com/i/JEGS/555/6302...CABEgJruvD_BwE
1971 Firebirds could be ordered with manual disc brakes from the factory. They used the D52 calipers. If you have access to older parts books you can determine which master cylinder they used. I used to know off the top of my head but that bit of information seems to have disappeared. I used to have one and those brakes worked well.
My 67 Suburban has D52 calipers with a 1" bore master cylinder and it stops well and it weights 5000 lbs. One of the best changes I made after going to discs was buying the best pad for my application. Not cheap (Compared to parts store pads)but, much cheaper than replacing my 15" wheels and going to larger rotors and multiple piston calipers. You will need to converse with a quality vendor like Hawk or Wilwood to get the correct pads for your usage and application.