if you just want an overdrive, the most economical way to go is to swap to the AOD. you can step up to the gear vendors over/underdrive unit that bolts to the back of the C4 trans, but the unit is more expensive than getting a complete AOD and converter from lentech.
I am waiting for my car to come back from the mechanic, he is putting in and AOD, buts its taking a while. He said that some of the parts from the kit i got won't work. The yoke to shorten the shaft isn't short enough and that there are other little parts that he needs..... about two weeks without my daily driver is killing me! I'm glad I didn't do it my self i guess.