Assuming 3 speed MANUAL transmission, I would try:
Standard O-ring #111 7/16"ID x 5/8"OD 0.103" Cross Section Nitrile
NPD Part Number : T-7288-2
OIL SEAL, Transmission Shifter Fork Cam and Shaft, O-ring seal,
Seals the shift shafts as they exist the case side
Manufacturer Reference #'s:
Fastenal: 1166823
Ford: 1A-7288-A, E0UZ-7288-A
Note: It appears the O-ring goes in groove on shaft BEFORE shaft is installed in case.
What say
@BigOrsnge ?