Cost to replace auto transmission? Feasible to swap to manual trans?

What is your question?

I've mistakenly trusted various shops with changing my oil and checking my fluids, but recently noticed a leak in my radiator, so I bought the part and asked a friend to help. While swapping in the new radiator, my friend disconnected the hose that runs from the transmission to the radiator (I didn't know it did this, I'm not much of a car guy) and he smelled a bit of the fluid that dripped out, around 1/2 cup. He said it smelled bad, burnt. After the job was done, I started the truck to move it and immediately knew something was wrong. The transmission felt like it was grinding. I shut it off and we checked the fluid level - it didn't register on the dipstick. We added 3 quarts, and it finally showed on the bottom of the stick, but my friend advised against adding too much, saying it might make a bad problem worse faster. Now it works, but I can feel it slipping a bit. Am I totally hosed? Should I replace the thing or try rebuilding it? Truck has 203,000 miles and I was hoping to keep it a couple more years. Time to replace the entire truck? I'd rather have a manual transmission, does it make sense to switch now?