I would think by now, you could tell if it was definately the trans slipping, by color, and smell of the fluid. Have you peeked in the pan yet?
The 6700 settling to 6400 is from the converter flashing. But thats also a "tell". Your 4500 stall converter is flashing 6700. So its WAY off, providing your trans fluid is still in good condition. Jester and Cobalt are spot on.
One thing that needs clarifying, is the gearing. The higher numberically, the easier it is on the converter, and will not slip/flash as high. Same goes with anything that makes it easier to get the car rolling. A 4.11 is much harder to get moving than a 4.88, so the 4.11 will flash higher, and slip more. Cobalts point is lower gearing (4.11) will allow the rpms to rise slower thru the rpm range, THATS PROVIDED you arent slipping. His converter choices are also excellent builders.