Aluminum master cylinder with...
Aluminum master cylinder with plastic reservoir
A similar calculation is valid on the caliper side with a larger piston providing greater mechanical advantage. For a four-piston fixed caliper, we would use the surface area of two pistons, not four. The pistons on both sides of a fixed caliper need to move only about one half the distance the one piston on a floating caliper does.
Using a master cylinder size that provides reasonable pedal travel and maximum mechanical advantage will optimize your brake swap. Overall, we're trading a large pedal movement with light applied force to very small brake pad movement with large applied force. We're also using a relatively small bore master cylinder with long piston stroke to feed larger bore calipers to make a much shorter piston movement.
Depending on your swap specifics you may or may not have sufficient room to fit a different brake booster, so check your diameter and available space carefully. There can be a wide range of master cylinders that may work for your swap depending on the flange mount and bore diameter needed. You may also find the fatter cast iron variety master or the thinner aluminum one with the plastic reservoir sitting atop. One shape may fit better than the other in your engine bay.
Wilwood manual brake proportioning...
Wilwood manual brake proportioning timing valve
Whenever you modify your braking system, such as the addition of rear disc brakes, it may be necessary to add a manual brake proportioning valve to the rear brake line. This can allow you to manually adjust the pressure to the rear brakes in a case where the rear discs may be over-powering and unbalanced when compared to your front discs. A manual proportioning valve such as this unit from Wilwood can be used to decrease the braking force of the rear discs.
Some brake line adaptation may be necessary. If so, there are all manner of adapter fittings available, even metric to SAE, and vice versa. You can also buy steel lines with metric or SAE ends, and a flared end can be swapped as needed if you have double flaring tools.
Custom brake lines with proper...
Custom brake lines with proper fittings
Another option when it comes to adapting brake lines is that specialty companies, such as Crown Performance Products, can custom build stainless braided flex lines to any length and with any terminating end that may be needed. These can be used to plumb the calipers on an axle, or to plumb from the master cylinder to an axle.
Brake line relief coils at...
Brake line relief coils at the master cylinder
Where brake lines come off the master cylinder and route down to points on the frame rail, you'll typically find relief coils. These are there to provide for some movement between the body and frame without having the brake line stress, and fatigue or crack. Also, When routing your own brake lines be mindful of securing them to the body or frame every few feet or so. When in use, they are subject to constant flex and vibration. The metal lines can fatigue and fracture over time if not properly staked. Needless to say, it's prudent to take care when working on or modifying your brake system. If you're not confident of doing a reliable job, refer to a competent mechanic for a safe installation.