That was me.
You have to grind out a little bit of the crossmember. There's already an indent where the stock 350 front diff goes across the frame right in front of where the driveshaft goes. When you get your 400AT diff, you'll see where it needs to be trimmed (hard to tell from my pics).
You need a 400AT front diff and front driveshaft. Once you grind the frame where the diff clears it bolts right in.
Your 350 axles will still work. I put in 420 Rancher disc brakes (bolt right on at the ball joints) while I had it apart.
You dont' hook up either of the speed sensors on the 400AT front diff; I just snipped the wires off the sensors to clean things up a bit, have to leave the sensors in place to seal up the diff.
You run the actuator (green plug) wires up, and ground the white wire, and hook the wire coming off of your switch to the orange and white wire. Hook the other wire coming off of your switch to the pink wire on your ignition switch (power only with key on).
That's it. Just remember doing this you lose the safety feature that is built into the selectable 4wd machine ECU's. On the selectable bikes if the two speed sensors on the front diff aren't reading the same speed the ECU will not send power to the actuator. This is to prevent damage if, for example, your rear wheels were spinning at 30mph in a mud hole while your front tires were stationary. To engage the diff in that situation wouldn't be pretty. You'll need to either be stopped or on hard ground with all four wheels rolling at the same speed to safely hit the switch You can disengage while moving with no problems.
I used a stock 400AT throttle assembly which has the 2wd-4wd switch. Take the cover plate off with the AT/ESP switch and put your 350 plate on top. You could also use a 05-11 500 Foreman or 02-04 450 throttle assembly to get the same switch.