Steve, unfortunately the governors are not interchangeable, If you are talking about the part that screws into the back of the o/d tail housing on the "passengers" side of the tranny.
a pics worth a thousand....heres the one for a 4 bolt side cover:
And heres a 7 bolt side cover governor:
The later model ones are notorious for cracking and breaking because they have a black PLASTIC top on them that doesnt have a chance when it meets up against a careless, clumsy parts puller/transporter. mine has a hairline crack in it and still works, if its any consolation. I dont know of many that have survived the past 30 years unscathed.
you can find em on ebay from time to time.....oh the guy stood me up for the od, ill go there on the weekend when its best for him