Got a new Allison AT540 governor to see if it would adapt to an TH425 transmission. It does! The Allison governor has no speedo gear but the gear from a TH425 fits on it. The Allison governor has no can as it fits inside the transmission in it's original configuration but it will fit inside a TH425 can. It does fit, shift and the speedo works. The Allison governors were available in several different rpm ranges to go with whatever engine was driving it. This particular governor is for 3000-3400 rpm and is a code 462. Sonnax makes them new.http://www.makcotransmissionparts.com/Allison-AT540.html
Part number on the Allison governor
Speedo gear swapped from the broken TH425 governor on to the Allison. I think the weights and springs could be swapped and then it would act like a TH425 governor.