Per my parts manuals, GM, at least back in the '70's did not sell weights and springs separately. They sold the entire shaft assembly as a service part. Here is an example for a Chevy HEI:
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1976 LeMans B09 Freeway Enforcer, 455/M40 Smokey
1977 Trans Am, 400/M21 Black/Gold Bandit. 44K actual miles
2017 Sierra SLT 1500 Z71 4X4
2019 Canyon SLT Crew 4X4
