Can you get a datalog of the dead spot? Maybe you can share that where you can get more eyes on it. There's some folks here that have experience with turbos(not me) that I bet would be able to help.
Edited to add: After you get the BOV right. That sounds like a likely candidate to cause an issue in a pedal situation.
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'65 Tempest 467 3650# 11.30@120.31
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