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You will need a TH400 or 4L80e to hold a big block. PowerGlide, TH350, or a 700R$/4L60 transmission isn't strong enough (torque rating) is too low. They can be made stronger with aftermarket parts but then the stronger transmission cost more than the TH400 or 4L80e to get the same gearing. Not economical to try and fight the design limits set by the engineers.