Transmission won't find a gear

So I swapped drivetrains into my current '68 Barracuda from my last '68 Barracuda (360/727). Everything worked fine in the other car.

I went to drive the car for the first time today and I'm getting nowhere fast. It will go into all gears and will move in all gears, but when I'm in drive about all I can do is idle forward. If I give it gas, it acts like I'm in neutral and barely wants to move. When I went down the street I pretty much had to let it idle back.

The engine and transmission were separated at one point before going back in, but I made sure I had fluid in the torque converter and then filled up the trans after a new filter. The fluid level checks just fine (right in the middle when hot).

I have a Lokar kickdown cable. I pinned the pedal to the floor and then pulled all the slack out of the cable and secured it.

The only thing I can think of that I could have done to mess it up is this. I had it on jack stands when I first got it running. I cycled through the gears to make sure everything worked before putting it on the ground. When I went to put it back in park, I didn't have any brakes (hadn't bled them yet) so the wheels were still spinning. It made a grinding noise obviously. Could that have ruined my trans?

I don't know a whole lot about automatics, so I need some help on where to start troubleshooting.

Thanks.