Transmission? Poor acceleration.

[telly]

Ok, I guess I'm ready to hear what I don't want to hear. Here's the story:

Had someone helping me to change the oil in my 72 C20. Turns out they drained the transmission and then added 4 quarts of oil to the engine. I drove it about 10 miles then started to feel a lack of acceleration. Got it back home and figured out what happened, so I refilled the transmission and changed the oil. Problem is I'm still having acceleration problems. What's the chance the transmission isn't damaged and not in need of repair? If not a transmission problem, what do you guys think?

Edit: I used Dex/Merc transmission fluid to refill.