Under 1966 K1000 PICKUP 5.0L 305cid V6 I found a starter that fits:
GMC 1000 SERIES 1961-1965
GMC 1500 SERIES 1961-1965
GMC 2500 SERIES 1961-1965
GMC 3500 1961-1966
REMY 25372 Premium Remanufactured.
Shows Alternate/OEM Part Number(s): 10455309, 10455344, 10465152, 10465320, 10496878, 1107371, 1107375, 1107376, 1107751, 1107752, 1108357, 1108483, 1108484, 1108752, 1109033, 1109076, 12304043, 12304988, 1977079, 1998229, 1998248, 1998470, 1998499, 1998594
https://www.rockauto.com/en/catalog/...ter+motor,4152
Under 1961 GMC 1500 with a 5.0 305 I found:
REMY 25372
Alternate/OEM Part Number(s): 10455309, 10455344, 10465152, 10465320, 10496878, 1107371, 1107375, 1107376, 1107751, 1107752, 1108357, 1108483, 1108484, 1108752, 1109033, 1109076, 12304043, 12304988, 1977079, 1998229, 1998248, 1998470, 1998499, 1998594
https://www.rockauto.com/en/catalog/...ter+motor,4152
Since I didnt have a year or anything to go on.