Transmission during engine break in question

[Mrturner1]

I have a 1970 c10, just rebuilt the engine and while I was in there I did a stall converter and a deeper trans pan, new trans filer and a new vacuum shift modulator. During the engine break in I let it run for 20 minutes and it varies from 2100-2500 rpm's. It never went out of Park obviously for a break in.

My question is this. Can you ruin a transmission while its in park? Does it heat up even when not being used (in gear)?

It's a TH400, now with a 2600rpm stall, and it had some fluid in it during engine break in, BUT it wasn't full by any means. I added 5 quarts just now after shutting it down and it finally showed on the dip stick. Now, I don't know what these transmissions hold as far as fluid, but it's also got a deep pan so it'll need more than stock.