<Loosen the nut on the brake pedal spindle undo it about 3 turns then get a block of wood put it flat on the nut and tap it with a hammer you will no the spindle has come loose as it will move inside the brake pedal then repeat until the brake pedal comes off and to get the rear master cyinder off you need to remove the rear wheel>

If the nut and trunnion lever on the inner end of the brake pivot spindle are removed, then the brake pedal with the spindle attached can simply be withdrawn from the outside (although having the pedal loose on the spindle can help when replacing the assembly).
There is no actual need to remove the rear wheel to drop the master cylinder and bracket, although that does give more room to work.

<there is a bolt behind the engine plate that secures the master cyinder bracket to the frame>

At some point (during '78?) the design changed so that the 5/16" bracket bolt fitted in a hole from the outside of the engine plate, that is how it is on my late '78 model.
With that assembly, there is no need to remove the engine plate to release the bracket although it is still necessary to remove the large pivot "bolt", the R/H footrest bolt and the bolt at the rear end of the bracket as well as the bolt that screws into the bracket through the engine plate.