Can i replace my 66 with a 400

can i swap the borg warner for a 400 easily

No, not easily. The problem with the GM400 is that it has an integral bellhousing, meaning it has a particular bolt and dowel pattern where it mates to the engine that cannot easily be altered or adapted. Reportedly the GM400 that fits a later 5.3 – and no other – can be fitted to the earlier engine, but not easily. The dowels are larger, so you need stepped-diameter dowels, and IIRC there’s also some issue with the way the torque converter fits.

This might be a job for John’s Cars, as they offer kits to fit a 350 transmission with their Quarterbreed line. Fitting a 350 might be better than the 400 anyway.

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