Do you use chamois cream? I have plenty of time in the saddle, but there's no way now that I would even contemplate a ride of that distance without it. Now that I do use it, I even use it on my shorter 20-25mi rides. It is a world of difference to comfort without.
Having ridden a suspension seatpost that was useless for myself at ~310lb, I'd be guessing that any such system would be near on useless at 350