There is one major reason why this will not work for you. The TH400 version of the Dana 20 has a special input gear to go to the (28 spline I think) TH400 output. There will not be a Dana 18 input gear you can use with this spline count. It would be a nice idea otherwise.
2004 Nissan, 1966 Buick Skylark GS, 1965 Chevy II Nova, 1955 Chevy 2DR Wagon project, 1948 Willys CJ2A Jeep My current project 4.8L Chevy Vortec LR4, SM 465, Dana 18, Dana 30/Dana44, 34" Swampers with 4.5" RE Lift.