placement of solid state voltage limiter on 1970 std dash?

You could mount your regulator anywhere including behind a kick panel or in the glove box with enough wire. You have 3 connections to make, 12 volts at switch on, 5 volts to the thermal instruments, and chassis ground. Switched 12 volts is blue with white tracer, at the panels harness connector. The 5 volt wire could be attached at any thermal gauge. The printed circuit board would conduct the 5 volts to the other related gauges. Ring terminal poked under a speedy nut works or doesn't work. Its iffy. Better to place that ring terminal on top of the speedy nut with additional toothed washer and 10-32 hex nut. Chassis ground wherever.
Alternative to that is force a small sheet metal screw into those slots where your OEM limiter was. Larger than #6 may crack the board.