Keep the engine tranny combo. I have a 49 Dodge Coronet with fluid drive. Less power than you have and I drive mine 55 to 60 mph everyday to work. I don't due interstate yet, mine is bone stock mechanically right now but that will change. There are a lot of sites on the web allpar.com, oldmopar.com etc. just keep looking. There are a lot of sources for parts out there check the links of all of these boards and to help you get started here is a page with several links.
http://www.50plymouth.com/11-sup/supp.html
