What is the transmission number?

Do you mean, which transmission was used in that truck? That would be the A340, if the mileage you posted is correct, I suspect it's time for a full rebuild. These are a very good transmissions in the early years like yours.
This isn't your vehicle, but it does show the basic process for transmission removal:
https://www.2carpros.com/articles/how-to-remove-an-automatic-transmission-rwd

This video gives you an idea of the teardown process.
https://www.youtube.com/watch?v=pSTeisqdiQY