I concur with everything that Nick has already stated. Here's another option for rollerizing the output shaft in the TH400, all in one kit. Sonnax offers their 34006-SPB TH400 HD Case Bushing Kit. I use this kit regularly on anything used in HD, 4x4 & definitely on snow plow truck applications...
The only time that I use a cork gasket is on an overhaul, meaning that everything is bone dry after a rebuild. On a trans service (fluid change) I'd only use the fiber/composite gaskets. It's been my experience that it's impossible to drain all of the fluid on a service. The residual fluid...