Build a 904 or buy one off the shelf?

I have a question similar to this thread. I have a 71 dart swinger with a 318 and a 904 automatic torqueflite transmission. I am new to owning classic cars, just last weekend I noticed that the dart when shifting from 1st to 2nd gear the car shifts then revs like it like it is slipping or something! I don't even know if it shifting into 3rd. Before I could distinctly feel the shifting between all three gears. Any ideas on what is wrong all of a sudden? And how much it should cost to fix?