If the converter is still from the Impala, it's not going to work with the L78 cam, as you are aware of. You'll need a converter to match the application, with a higher "stall" speed, either a rebuilt original or an aftermarket tuned to the cam.
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Mitch
1970 Chevelle SS
1966 Chevelle SS
1967 Camaro ss/rs
1938 Business coupe, street rod
2000 FXSTS, original owner, 13k miles
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