Let's first be sure you understand what your amp is and what you are asking the amp to do. The mc250 is 2 amplifiers each with their own input (preamplifier), left and right channel which work separately when ran in stereo. Mono is to run the mc250 as 1 amplifier so what you are doing is combining the 2 amplifiers together, but since they both have their own inputs(preamps) only 1 is needed. So your mono/stereo switch switches the mc250 from using 2 inputs to only 1 input (mono)and sends its lone signal to both amplifier sections which also need to be jumpered together at the outputs to combine the amplifiers together. When you jumper the outputs you are paralleling the autoformers thus halving their output impedance. So when you jumper the 4 ohm taps you are creating a 2 ohm tap,when you jumper the 8 ohm taps you are creating a 4 ohm tap and when you jumper the 16ohm taps you are creating an 8 ohm tap.
as Mr. Shank stated, you need to jumper the output tap your speaker load requires and attach you speaker to it and a com(mon). Place the stereo/mono switch in mono and use whichever input the amp calls for for mono use.
Disclaimer: I make and sell Buffers. I also sometimes Modify, Build and sell preamps and amplifiers. My opinions are not intended to be sales pitches. I am a one man shop and prefer to spend time with my wife and family, i work full time on call and love to spend my spare time doing other things. I only make, modify or build things for those that seek them.