It's been a while since I looked at this thread. VERY nice! I really wish I had a TIG welder and machining tools like you do.
When I saw those pictures of the board jacked up on your couch and with both trucks turned so hard, my first thought was that the axles had to be driven independently. A fixed axle would create issues with turning radius. Why not drive all wheels from their own independent motors? It's not like you lack the room on those giant trucks. With some minor added circuitry, you could even account for steering angle so that the outer tire gets more throttle than the inner tire in a turn.
Regarding Castle ESC's, you mentioned getting 300 amps out of them before. I assume that was for a couple of seconds and that you are referring to phase amps right? With such a tiny heat sink, it is surprising that 160 amps is even possible without them getting quite hot. I've only ever owned a single Castle 160 and I burned it out on a small scooter I was going to use it on. After that I went back to real EV grade controllers. If the 160 could handle halls, do regen, handle an analog throttle and a few other things that you commonly get in EV grade controllers, they could be a real contender in my mind. After burning out that one ESC...I've just stayed away from them.
This build is pretty amazing. I love all the custom work! My guess is that the turning angle will be a problem as it tips over the board so far which makes staying planted on the deck rather difficult. My guess is that you will try to straighten that up a fair bit so that the deck doesn't tilt so much. I look forward to seeing this project getting powered.
Any idea how fast you want it to go? 48 volts ought to get you 40mph easily...depending on the gearing and motor Kv.