Look, the bottom line is your boots shouldn't have a smaller foot print than your bare feet. Stand on the board in your socks and see how much overhang you have. This is your baseline. Add a liner and rubber sole, and it's going to get worse. I don't put a lot of faith into brand sizing, since it's a known marketing tool for boot manufacturers to "size-down" their molds.

The number one factor should be comfort. You might have some luck with your existing boots by shaving the heel and toe down on the outsole. You might also consider increasing the angles on your stance for less overhang.

Truthfully, I think your predicament is finding a wider board.

...seems a bit drastic. what type of liners are you dealing with? i downsize a full size in a boot. 2 sizes may be impossible. if it's a foam style liner you can try pulling liner, putting it on foot, locating the pressure points, and making small incisions (star shaped) for room to push out. this leaves material for that insulation aspect. i've done this on my salomon liners and it's worked well. 2 sizes maybe a bit much though brolmes. if you have any more questions hit me up, i can fire you fotos on how i go about downsizing...