GMC usd a long push rod until the mid 50's and went to a shorter push rod. The older lifter has the push rod going down in it and the newer style has a cup on top for the push rod and we call them the milk can lifter. Post a picture of another lifter. Both styles appear on eBay quite often. Used push rods are quite plentiful. Kanter is like most vendors. The guys that started the business knew their business. The people they hire now just don't care.

Rock auto probably still has what you are looking for. They still carry the milk jug lifters. Used lifters need to be resurfaced to make sure they have a crown or they won't spin. An old Sioux valve machine will do that, but few people know how to operate them.