Very easy retrofit, esp if you have a full car to pull from. The wiring can all be handled at the AT shifter connector. Reverse lights are the only thing that will take any mental fortitude, but the solution can be as easy as a manual switch. The pin out can be found searching this forum or in a service manual.
Years back, I converted a 1990 fwd 3AT loyale to fwd 5MT, then to awd 5MT DR from an RX and 5 lugged it. The pictures are long gone but the post is still alive from my old defunct account:
I live just down the hill from Lake Tahoe in Gardnerville, if you ever need anything. I still have 2 RX's and a garage full of parts, including factory service manuals.