So I was trying to accomplish the same this as the OP with the least amount of headache. The car is a 70 Dart with a moderately build 318 and a 904 that I am building for long distance road trips. After I did a bunch of research, I contacted Rick at A&A transmission in St. Louis. We came up with this: in the 1980s Chrysler was trying to chase gas mileage in their V-8 cars and the rear end ratios got down to 2.45:1. Which was fine on the highway, but a real dog around town. So they changed the internal 1st and second gear ratios from the old 2.45, 1.45, 1.00 to the new 2.74, 1.54, 1.00 making 1st and second gear act like it had a lower (numerically higher ) gear ratio. They also came up with lock up converters - first using hydraulics in the trans with serious problems and then with electronics connected to a computer. What A&A is going to build me is a 904 with the new ratios and a lock up manually actuated with a switch (and some fail safes to keep me from dying at a light because I forgot). I'm going with 2.94 rear gears which in 1st would be like the old trans with 3.28 rear gears, 2nd would be like 3.10 gears and 3rd would be 2.94 and lock up on the open road which should be worth another 2-300 rpm drop at 65. Like I said this is all in process, but no new drive shaft, not tunnel cutting, run some extra wires and off we go (theoretically).