Congrats on the cruiser,
It wont help you much to put the 3spd case on the 4spd trans. If you want lower gears you have to go with the gm465trans to the 3spd case, But you will have to get the adapter for this. I like the 4:01 Orion t/c be hide the 4spd trans because there is no changes to the rig to make it work. And in the long run it is cheaper. The fj62 is a go way to get every thing you need to make a good trail rig. You can us the axles and drive shafts and all the brake hardware. The only thing you would have to do is, out board the front springs hangers and that is not all that hard to do.
All the info you will needis on web, just search