Convince me to put a TKX in my Dart

There is no reason at all to cut out the top of the torsion bar cross member and not replace it with something else. I would not rely on just the bottom trans support to maintain strength in the crossmember. You can build (or buy) a hoop across the top that will maintain the strength of the original and give you room for a larger trans. The subframe connectors and torque boxes strengthen the frame in the wrong plane to help the situation. Seriously think about the weight of the entire front end of the car and that crossmember supporting it.