One of 2 things. A Mitchell Plate to ball head adapter or the mounting plate that is a single 3/8" bolt. If you get the Mitchell plate>ball head adapter then you can do quick adjustments for leveling.
The Bogen/Manfrotto accessory page should have what you need or look at the B&H site.
Also some of the Hollywood style custom production houses may have something else for the 75mm size ball.