If it pulses the break pedal and the steering wheel it is the rotors being warped, but you described it a little differently. A glazed rotor will not grip the pad well causing it to be hard to break. One thing that can cause problems such as you described is hot spots, small glazed areas in the rotor. So it will grip-slip-grip-slip, similar symptoms as a warped rotor. Measure thier width and if they are above spec have them turned with a fine cut. Warped rotors, glazing, and hotspots are caused by two things, incorrect/uneven lug nut torque but mostly excessive heat. Sometimes a sticking caliper or caliper slide can cause this also. Either way your getting excessive heat so either your riding the brake, waiting until the last 20 feet to stop, or there is a probem with the rear brakes. Mostly they are just out of adjustment. I adjust them from the inside until there is no play between the rotor and shoes.
The 4 lug rear brake conversion is off the T birds and SVO mustangs.