I don't think you need to be worried about the strength of the Toploader four speed. They are found behind stroked 427 Fords and larger engines. You can ask David Key whether you need the big input and mainshaft but unless you are running in excess of 600hp or a big shot of nitrous I don't think you need to wory about it.

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Paul
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retired mechanic after 35 years
specialized in Holley carbs and Ford Automatic Transmissions