The only way I know of to ensure there is no dirt in your trans, is to take it all the way down and clean the components and the case. You need some special tools to do this. Some can be 'home made' but you also need experience to know the disassembly/assembly process.
Most all the Ford transmissions built upon the generation before. Therefore, many of the internals are the same from one rebuild kit to the next.
For reference, check out my Cruise-O-Matic rebuild.. CLICK HERE