Sounds like the gauge doesn't send current until it senses a ground. I think you are back to the sender not grounding in the head. You went from cast to aluminum too. The head should be well grounded thru the head bolts, but never say never if the builder put sealant on those bolts too.
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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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