as zman said, but they are expensive, you'll lose your factory kickdown and shift pattern. why not just rebuild the hydromatic aka jetaway?
I had a 331 with a hydromatic in my RPU. The ****** was fine, but it was HUGE! I ended up getting an adapter from Wilcap to put a much smaller 700R4 behind it. It was either that transmission or me inside that cab, and since there was another option for a ******...