cutting back of dash

I have done this a LOT, and yes you can cut the board as long as it doesn't interfere with something else (a gauge, light, or power supply to something)
I have even cut the board back for aftermarket gauges and used the old copper tracks as solder points to wire new gauges in so they all still use the main harness connector on the back of the cluster as a disconnect point.
Also cut the metal housing to make room in the back for electrical and gauge fitting, and all the lighting for everything (cluster back lighting, gauges, turns, high beam and signals all LED.)

You can tell this isn't stock.
All three gauges in the left OEM holes are mechanical aftermarket (Volts, Temp, and Oil pressure in both cars.
The OEM fuel gauge in the 72 (top pic) has been modified and moved to the far right where the oil and temp lights were.

The first pic is the Wife's 72 cluster and the second is my car.
The fuel gauge on the right in that pod is even connected using the original connections on the back of the cluster.
Also, in my car (bottom pic) you will notice the oil and temp lights got replaced by a clock, because I need to know what time it is frequently during the work day.