904 Transmission's demise?

My Duster has about 185K on it. I bought it from the origional owner so he knows the history. He says that he rebuilt the engine, a 318, which he appears to have done. He did not rebuild the transmission though. He tightened the bands he said.

It shifts fairly quickly and firmly into 2nd and then goes into third quickly (as in speed) but not nearly as firmly. I wouldn't say it slips, but It is not real firm like the 1-2 shift.

1. Is this early shifting/firmness difference a sign of the demise of the 904?

2. If not, is it acceptable to put a shift kit in it at this high of mileage or will I damage it?

The car is not a DD and I would like to get through the summer about 3K miles or so and then I am going to build a new combo for it.

Thanks as always, ROB