Damn that sucks to hear man. Well if it helps I have done a bit of searching and calling on the 6L80, when I first got my truck I wanted to know all about it and what to expect. The best builder I know told me he rebuilds a lot of them and see's failures from the pump as low as 100k miles so I asked what to expect on pricing if that happened to mine. They are 3-4k for the trans alone. I was freaking out. He did say something other builders said too, that they don't even usually bother repairing them because it cost the same to fully go through one that it cost to just buy a GM or Jasper unit.

The good news... I started digging more and found that you can get low mileage used 6L80 transmissions in scap yards for around $1200 at most yards, they were all 1200-1300 except at LKQ which they want 1800... That is likely your cheapest and best option. I found a lot of them with only around 20k miles too for $1200

I guess none of it good news, but buying used can save you a ton of money and a used low mileage GM trans is still an excellent choice.

Make sure to flush your lines really really good, there may be metal in the fluid.

 

Either one can damage the pump. Was the old motor locked up?
Sometimes you have no choice but to pull the converter with the motor.