My 763 was level to the axle tubes and up to the pedals up front. I removed the front covers by the pedals and the covers at the drive motors, tipped the cab up and dug out all I could through the holes. Next I poked holes through the drain holes at the rear of the chain case and commenced pressure washing. It wasn't easy, nor was it fast but it is now spotless down there and takes very little to keep it that way as long as it isn't let go so long again. Another benefit of having it clean, any leaks show up right away rather than being held back by the muck.
I think unless you have a lot of time on your hands and were planning on a complete restoration project, tearing down to the tub would be a lot of work.
Oh yeah, depending on what you have for a wash area, you might want to dig a hole and park your machine over it so the drain holes empty into it as a lot of nasty crud is going to come out of there.