What was uncomfortable about the CX bike on that ride? Honestly, I've never ridden a 400K (my max is an imperial century), but I'd imagine anyone would be sore after that. What size tires are you running on your CX bike? The fastest way to comfort is fatter tires and lower air pressure. I think your CX bike probably has far more clearance than most vintage frames. The next thing would be adjusting your fit and getting your controls comfy (saddle, saddle height, bar shape, bar position, bar height). The long distance forum would be a good place to ask.