1. Disconnect the GREEN or GREEN/WHITE wire from pin 4 of each output tube and insulate the bare end so it doesn't short out against the chassis.
2. Solder in a 100 ohm 1 watt resistor between pins 3 and 4 of each output tube.
You are now in triode mode. The amp will now have only about 60% of the power it had before but you may like the subtle change in sound. The amp will have a little more mid range projection. Singers will move slightly forward in the sound stage. The extreme top and bottom end of the frequency spectrum will be rolled off very slightly from what it was in pentode mode but you probably won't notice any change in frequency response. The amp will sound a little "smoother".
You can also install DPDT (dual pole dual throw) triode/pentode switches and make the move from triode to pentode easily switchable. The only problem is where to put the switches. If you replace your stock driver board with the VTA driver board the choice is easy. You don't need the stock bias potentiometer with the VTA so just pull it out and you now have a nice 3/8 inch hole which will be suitable for installing your DPDT switch. If you have a stock driver board then you may be able to find a place for the switch(es) inside the chassis. You could also drill a couple of extra holes in the chassis somewhere. Get a high quality switch - one that can deal with 400 + volts DC. The diagram below shows a switch installed for one channel of an ST-70 which would also be applicable for the mono Mark II, III and IV.