Drivetrain Questions and Suggestions

The 727 is a more durable transmission but it takes more HP to operate. A mild 360 and a well rebuilt 904/999 is a good combination. The 999 is physically the same in appearance to a 904 except that they were built with a 2.75 first gear. You see, in the early 80s, Chrysler was still a few years away from releasing a real 4 speed automatic with overdrive. Their holdover idea was to use the 904 with a lower 1st gear while equipping the cars with a taller axle gear. This gave the car decent low speed acceleration with a lower cruise RPM. I do not recall the exact year that the 999 model was introduced but I suspect that it was 1980 or 81. One advantage to using the 999 is that your cooler lines, shift linkage and drive shaft that you have now will still work.
I have used these 999 transmissions before. I had a 73 Duster that had a dying 904. I replaced it with a 999 from a wreck and I instantly felt the improved accelleration.