Aftermarket sending unit not the same

sim issue here, 72 val 318 car. put new unit in from YO. sender would not work. checked ohms all good. grounded unit outside tank (grounded to fuel line with ground strap) and moved float through motion. worked good. put back in tank. will not work. checked ground all seems good. bought new unit and it is doing the same thing. worked fine before. old one real rusty, and found hole in tube.
hope I am not hyjacking your post, maybe this will help us all.