Sounds like its going to be a beast! Looking forward to the build pictures. I have my FLM kit just sitting on the bench and its killin me. I'm traveling like crazy for work right now and just cant get any time to build it.
So seeing your build will have to inspire me.
As for the locking the tranny question... If you are going to lock the tranny and use motor brakes then there is no reason for the 3 speed tranny. If you want the 3 speed to work, you have to use the machanical brakes. I have been running the stock 2 speed tranny in mine with no problems at all. The 3 speed really isnt necessary. You could save yourself a couple dollars and not do the 3 speed.
My pesonal opinion is screw reverse unless you run in tight spaces that have alot of trouble.