I think the Magnaflow spun cats are fine, in terms of construction, cell count, etc. The substrate length is less than the body. Its pretty easy to check by inserting something into both ends till it touches the substrate, then measure the gap. I'd be surprised to find a cat with less than 3".
This popular cat was specifically targeted when the allowance was written. They are minimum of 200 cpi, and none of them that we were aware of had bricks less than 3".
Yes, body length = substrate length. The substrate is housed in the cylindrical body of the cat, while overall length includes the tapers on either end.