The 42RH is a smaller case and uses less HP to drive it. Problem is in stock config it isn't very strong. You can build it to handle the power. It just costs more. a stock 375hp 440 is easily doable with a hopped up 42RH. Another nice thing about the 42 is the better gear spread, first gear is a 2.74 vs. 2.45 first gear. Second is 1.54 vs. 1.45. OD is same on both at .69. I am building one to go behind a small block that should have about 400hp. SMR is a great company to get parts/info. Also PATC has parts and reasonable prices on torque converters, I just ordered mine from them.

Here is a great thread on the whole install and comments/questions...and of course wiring it up. GDonovan here has a great schematic too. http://www.cuda-challenger.com/cc/index.php?topic=84774.0