I haven't done behind the factory lens of the 71 Rallye dash...but I have modified it without opening holes but adding into the dash pod itself. Modified a 72 B-Body Rallye dash a few months ago with Autometer gauges. Speedo/Tach in the big pods and replaced the two on the right with Oil Pressure/Water.

Looks great. I found the best way to mount the aftermarket gauges is to get PVC pip and cut 1/2" rings and jam it into the hole and then JB Weld it from the backside, use the included hardware to lock it into place and wire it up.