Finding a suitable strain gauge should be straightforward. While the PressureTrace can record the information, it would take some work to calibrate it for this use, because it's set up for inputs assuming a round steel chamber, etc.
The simplest set up would use an oscilloscope. Excite the strain gauge with some battery, use a couple of resistors to form the bridge, and have at. The little rod set up could be calibrated by pressing it on a bathroom scale to, say, 50 lb.
Do you know anyone there with access to an oscilloscope? A buddy of mine owns one, but no local shooter I know has a '73 (and certainly none who would loan me one for such a test
It might be possible to put a strain gauge directly to one of the forward toggle links. A couple of assumptions have to be made here, and calibration isn't as easy.
Also, your basement set up could be used after fashioning a new breech, say a tube with a strain gauge on it, or a cantilever plate with the gauge and a firing pin hole.