Unless you are
a) up for a huge challenge with little chance for reward or
b) are just a glutton for punishment
skip the SCX brand cars. I have tried to prep two of these (A Volvo and a BMW) for touring car proxies and found that the level of changes required to the body and chassis are simply too great. At a minimum you are looking at replacing the entire guide post assembly, modifying the motor pod to fit a more traditional motor, and probably modifying the interior to clear a new motor (which starts to bump up against the rules).
As Ary pointed out most of their cars are very narrow, although some would argue they are actually closer to scale than other brands. The one exception is their DTM series of cars, which are also wider in 1:1 scale. (Think EU version of NASCAR with left and right turns.)
Build it strong.
Keep it simple.
Make it work.
(Leroy Grumman)