No matter the trans, you're looking at getting a JW or whatever conversion bell housing to adapt to the big block. The 518/RH transmission take the same "hop up" parts as the 727. Better plates and hard parts are also available for the OD unit. A&A Transmissions are also Torqueflite experts. If you have the wherewithal to do your own trans, it will be relatively inexpensive. I have been buying parts/kits from a place called Transparts Warehouse. They carry name brands and have fair prices. They have an all inclusive kit for the RE/RH transmission, with the best plates and performance parts. Good parts and valve body mods get you a long ways on these.