No problem, since its a PS3 stick, there are no after market custom boards right now that will deal with modding straight from ps3 to 360.
You'd have to dig for a common ground circuit board for the xbox, like the easily accessible madcatz brawlpad.
The bare basics of a dual mod is as follows
1) Make sure your boards are common ground
2) Link the power and ground between both boards as both boards must be powered at ALL times regardless of what system is working
3) Link all the signals together
4) For a PS3/360 mod using the same USB cables, have a dpdt or any form of switch to toggle between the D+ and D- data lines between the two boards
As for specifics, I don't have a V3 SA board with me, but i do remember that one side of all the face button connectors is ground, and one is power. Use a multimeter with a connectivity function to tell which is which.
As for the select, and home buttons, i don't remember which point they are either, but you can unscrew the top face panel pcb and see on the other side of the daughter board that they are labeled. Trace these wires back to the main board and use those as the signal lines for those two buttons
As for the switch, you can drill a DPDT switch to the inside of the cord compartment or use an impV2 switch which toggles between consoles by holding a button when you plug it in.
As for finding a place for the pcb, i always dremel out that huge plastic post and mount my brawlpad/fightpad/4716 pcb diagonally where that post used to be
I make it sound easier than it is (well it is easy, just people have a lot of cold feet and don't want their sticks break when they do something.