All you need is one book. The ATSG Service Group for the 727-904. It shows how to build both. They are the simplest three speed transmissions ever in the world to build.
A Chimp could be trained to put one together and I am not kidding. They are that simple.
That said, in your position, I would keep the 904. As said, the 727 requires another converter and the drive shaft to be shortened.
The 904 takes way less HP to drive, resulting in more power to the ground. All the more important when working with a small engine like a 318. I would not steer you wrong. This is totally something you can do. If you can do an oil change, you can build a 904.