Capt is right on the HD 6-bolt torque converter cover. I've never considered it necessary for HD use though.
And the TH-400 is without a doubt the simplest three speed tranny to overhaul. Read up on it and do it yourself. TH-400 rebuild manuals are easily available from your local Barnes and Noble, Amazon, or eBay.
The very first automatic transmission I ever rebuilt was a TH-400. It set in my shop for about 15 years never having been used. I finally sold it to a friend of my oldest son. He put it in a mud truck and ran it for a couple of years without the cooler being hooked up.

It is still alive as far as I know.
