Sumos Springs has an application guide on their website to help you figure that out:
(E-350, not F-350)
Fronts are either
SSF-106-40 500lb capacity at 50% compression
SSF-106-47 1000lb capacity at 50% compression
SSR-106-40-1 or -2 2600lb capacity at 50% compression
SSR-106-47-1 or -2 4600lb capacity at 50% compression
Not sure the differences between -1 or -2 on the rears (I've only installed the fronts using the 500lb model).