No reason to box with what you are doing so far. I put a BBC in a TF and used smaller "C" channel about 10" long as a reinforcing backer at the engine cross member mounting tube to frame locations. Fitted inside stock frame "C". Not welded. Spreads the load. Big washer. Don't know if I needed it or not.
Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn
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