I'm really confused as to why no one has just made a retrofit sending unit for GM. They were roughly all the same short of depth, would cover 30+ years of every vehicle line they made probably, integrate a fairly effective baffle/sump like the later model sending units, and it would probably sell a ton of units.
Doesn't make any sense to me that the manufacturers choose to make everyone who buys their product cut up their stock tank, instead of making their product fit the existing applications which number in the millions.