Joe,if you want to use your 104 fronts you can use the stock pins.
there will be some extra pin sticking out of the top of the 103 arms but the 104 rims should clear the pins.. Just set the grub screws on the knuckles to proper ride height and use any needed shims under the knuckle to control knuckle droop.
Or you can run the spring on top of the knuckle (under the top of the arm) like the current 103 upright set uses.
http://www3.towerhobbies.com/cgi-bin...?&I=LXGJ85&P=Z
I would suggest buzzing down the front tires to get them closer to 103 size.
Btw.. Mark has been running 103 front width and 104 rear width (rear tires) to much success. The key is getting the foam small enough to control the traction roll issue.