The plug you are looking for is on the passenger's side of the transmission. It has a square head. Remove the plug, The proper level is at or just below the level of the plug. Don't use GL-5 lube. It is not safe for brass, which is used in the transmission's synchros.
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12
'52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.
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