Hi Trevor,
Cool application! Your concept of operation is correct. All current measurements on a cFP-AI-110 module are voltage measurements of the voltage-drop across an internal 100Ω resistor. Take a look at page 6 of the
FP-AI-110 and cFP-AI-110 Operating Instructions for more information. A current measurement of 40A would destroy the unit instantly.
The cheap way to take a current measurement of 40A with power resistors is to put a few identical power resistors in parallel. The current should divide evenly between them, and you may then take a voltage measurement with the cFP-AI-110. The caveat is that your generator must not regulate voltage, i.e. the output voltage must be linearly proportional to the output current with a purely resistive load. I can't recommend a particular power resistor, but I found a few online for about $10 / 100W. You'd need five of these at least.
There are also other options. If your signal is AC, you could use a transformer to step-down the current. You could also use a converter to bring your signal into a measurable range. You could use an analog current probe. I hope these ideas help. Let me know if you have any additional questions.