Transmission seized I think.

[CreativeNotice]

So a friend of mine was borrowing my '82 C20, 350 with a powerglide. While driving he hears a loud "clank" and loses power. He manages to go a few more miles (eeek!). Now that it's parked here's the symptoms:

1. When in park I try to crank the engine, the starter does try to spin but I just hear a clank and the engine doesn't spin at all.
2. When in neutral the starter doesn't even try to spin.
3. When in park and I push/pull the truck I can hear a clank.
4. Transmission fluid is leaking.

One last thing, a few months ago I dropped the transmission pan, replaced the filter and pan seal. I did this because it had the original seal on it and had a fluid leak. After the maintenance there was no leak and it ran fine until, The Event.

I'm going to take it up to the transmission shop and get their take but I don't know a whole lot about transmissions so I'd appreciate your input so I have some idea what the issue(s) could be.

[HAULIN IT]

Your post leaves me with more questions than answers.
1) What in the world are you doing with a Powerglide in a 3/4 ton truck? That thing has to be a real slug!
2) When you said "push/pull in park" are you saying it freewheels in park (as in, it would roll down a hill if the E-brake wasn't on or a block at the wheel ) but clunks?

My first thought would be to remove the torque converter bolts & then see if the engine turns then. Unless there is something bound up in the converter/input shaft...the engine should still turn over. Lorne

[CreativeNotice]

You're right, it's a TH400. I just took a photo of the pan and compared it to this identification post http://forum.73-87chevytrucks.com/smforum/index.php/topic,11124.0.html
When I say I can push/pull it, I mean it will roll about 4 inches before stopping. When it stops I hear a clunking sound.

[Captkaos]

You won't be able to move it if it is park...
What does the teeth on the flywheel look like, the starter not engaging could cause the clank.  If it is engaging do you hear it binding trying to turn?
I agree on removing the torque converter bolts and seeing if the motor will turn freely.