Here's what I see:
Master Cylinder looks like a Chrysler. It will may be mounted to the firewall with an adapter plate (original 4 stud or bolt to 2 stud or bolt)
In the second photo, it looks like the brass distribution block where to lines from the master cylinder go. Chrysler calls it the Safety Switch, or a Combination Valve if it has additional functions.
The object with the Wilwood written on the knob looks like a Wilwood adjustable Proportioning Valve.
The small brass block in front of the proportioning valve could be another proportioning valve or a metering valve.

Lets go through the devices before touching on the issues:
The Master Cylinder's bore and stroke effects the amount of hydraulic fluid moved for a given amount of pedal travel.
How much movement and force at the calipers will depend on caliper piston size and number.
Therefore you need to figure out what calipers are on the front and rear to see if the master is appropriate.
If the master cylinder is appropriate, or you change to another one, if the masters were originally intended for a disk-drum application, there may be a low pressure check valve in the outlet for the rear. This can cause additional dragging on a disk as there's no springs pulling the pads back like a with drum brake. This is not your current problem so don't worry about it for now. Just something to keep in mind when adapting master cylinders to rear disk.

The Proportioning Valve does nothing until brake line pressure reaches relatively high pressure. Something around 300 psi is typical. Then it proportionally limits the increase of pressure to the rear brakes.
For example if the you begin to apply the brake for a stop light and pressure in the lines is at 250 psi, both the front and rear will be at 250 psi. As you press harder and the pressure from the master cylinder goes to 300 psi, all brakes still get 300 psi. But after that, the front will get full pressure, lets say 350 psi and the rear a percentage less, say 325 psi.

A proportioning valve is used to prevent rear lock up during hard braking as the weight shifts off the rear. Its very common with disk/drum combinations because drum brakes are self energizing. In other words they help themselves apply more pressure to the drum.

A Metering Valve is used on the front brakes of some vehicles to hold off the intial application of hydraulic pressure. Doing so makes sure the rear brakes start to act just a hair before the fronts, reducing the chance of front lock up on slick surfaces.

The Safety switch is just a little shuttle valve between the front and rear hydraulic circuits. When the pressure is unequal, the shuttle moves out of position and closes the switch. You see the light on the dash, and say oh Sh1t! which you probably already said and didn't need the light to tell you that.

Starting in 1970, Chrysler combined some of the valves into the Safety switch distribution block.
Look through this booklet for pictures and explantions. http://www.imperialclub.org/Repair/Lit/Master/274/page08.htm

Soft pedal could be air, a bad hose, something about the stroke. It would not be related to the proportioning valve function.
It could be related to using a master cylinder that doesn't move much fluid relative to the size of all the caliper pistons if the pistons have pulled back.

Extra braking force depends on what you are comparing with. Drum brakes require less pedal force to get the same stopping power (compared to disks of the approximately same size).
Friction material and surface of the disks also factor into the bite. Sometimes changing linings can make a noticible difference.
Diameter of the disk can make a difference. A caliper located further from the center will have more effect than the same one on smaller disk.

So again idea to see what disks and calipers are on the car. Would't hurt to pull the pads and see what they are too. There should be codes stamped into the edge of the lining or printed on the back of the metal plate.