The 103 tyres can be run on the 104, sure, but requires a few things to work.
-Overall ride height needs to RAISED (because the 104 tyres have a wider diameter, when you put 103 tyres on the car the wheels may not even touch the ground).
-Front wheels need different bearings. 104 uses 10x5x4 bearings. 103 rims need 8x5x4 bearings.
-As mentioned already you need to put some form of 103 typre front end on the car (either 103 front end or a 109 front end) to make the wheel track or whatever its called right. Otherwise your rear wheels will be waaaay wider than the front.
Other than it should all be good.
EDIT: Was jsut wokring on my car and forgot to mention that you will have to use a f103 style front end with f103 front wheels anyway. The 104 front wheels will not fit on the 104, the king pins are too big.
Last edited by LordAnubis; 03-08-2011 at 12:31 AM.