Bill,
Sorry to hear about your problems. It depends on how much of a rattle that the convertor is making. They do have some play, but not much. I once broke the impeller from the splines that drives the input shaft on a "Vega" converter, so it can happen. When it broke it sounded like a gunshot. On the Turbo400, I doubt if you broke the input shaft or the planetaries, but you can usually take the trans out of park and there is usually enough drag that you can rotate the input shaft and watch the output shaft rotate. The other thing would be a complete loss of the front pump.
Does your car have a spool, a true posi, or a limited slip rear end? I had a limited slip in a Ford 9" that broke the edge of the carrier and it would not move the car in any gear. I thought that it was the transmission at first, until I saw the driveshaft spin and the car not move. I have also seen with the limited slip rear that the axle spline strip and it would not move the car.
Bill Mitchell