Adjustable High current source (40A?)
The circuit shown is indeed a plausible place to start.  However, I built one about 10 years ago when testing military robot batteries.  They only needed 10A discharge rates, and I was using 6 big MOSFETs in parallel. Each FET needed an equalization resistor in it's source leg before they all connected to the current sense resistor.  this is because the gate threshold voltage is slightly different for each MOSFET and so, as I saw on my thermal camera in real time... the one with the lowest threshold turns on first and starts to heat up.    Gate threshold voltage drops as temp increases on a MOSFET so, the first one overheats, ops, and the second lowest turn on threshold part starts to fry itself. :-)   So, You need to make sure your series resistor is large enough to get everyone turning on no matter how much shift there is in the gate threshold.  It also helps if you look for FETs that are designed for use in the linear region. [though they are more expensive and harder to find.]  Also, use a separate op amp for each FET with some series resistance driving in to the gate to avoid too much capacitive loading on your op-amps.  Good luck.  on another note, have you considered just using a current limiting power supply?