That's a good question, the drawing doesn't show a ground I suspect they are grounded through the positive lead.
Pull a plug lead and measure from the cap to chassis, it's about 8K from memory. Then unplug the coil and measure it again
it's either going to be 8K if it goes to ground or infinity if it goes to positive wire.
BYW check what type of High tension lead it has, mine started to miss and I found it had both a 5K resistor cap and the carbon
resistor wire which was arcing at one end, You should have copper wire with a resistor cap or resistor wire with a Non-resistor
cap, not both.
Adding a ground to the bracket certainly shouldn't hurt anything.
I will have a look at mine later.