Rally cluster in a base 70 swinger dash frame.

Yeah its been done. Total cost for one member was reportedly around 1200.00 That would vary depending on condition of the rally panel you start with, etc..
Cutting the standard dash opening larger isn't the main issue. Adding metal where there isn't any is an issue. Those few screws across the top of your standard panel are in different locations from those in a rallye panel. The metal where those screws find U nuts attached isn't continuous.
If you can conquer that, small differences in the wiring and such aren't such an issue.