I recently had to work on my (new to me, Fordson major Diesel) because the rack was stuck.
No problem removing the "SIMMS" plate, just try to keep things clean and I did not loosen the metering levers, just cleaned the rack and made sure it slid back and forth. Lubricated it, cleaned out the fuel shut off and extra fuel "button".
I also took apart the governor and checked the below to make sure it did not have any tears. Be careful if you take that apart, there is a small spring and check ball that you do not want to lose.