A-999 behind a 340

I'm planning on that exact thing. If you read the MP Chassis book, old Larry Shepard will tell you the 999 is the hot ticket.

Just know that most 999's were lock up which may or may not be what you want. Not so simple to change to a non-lockup, many specific parts and many variations to the transmissions as the years went by. The 999 was non lock up until around '78. Hard to find those particular transmissions laying around now though. Seems that guys piece them together using the desirable parts from later units.

Head over to Moparts and type in a search for A999. Make a cup of coffee and start reading, lots to sift through!