I've been down this road personally. The pony handle will go on a standard shaft if you really push them hard into place, then you only get a couple of threads on the screw (if you use the chrome decorative pieces and plastic washers that mount between the door panel and the door handle.
If you look at an NPD catalog, you'll see there are 2 different mechanisms for opening the door. One is for a pony, one is for a standard door handle. The standard has a shorter shaft.
If you don't use the decorative chrome piece and plastic washer, you might be okay. But with them, in time, the splines wear down and the handle comes loose due to a poor contact surface.
I don't know if there is an extension that you can add to your existing door mecanism or not. If so, that might be the cheaper way to go. If not, you really should do it right the first time in my opinion. Otherwise, you'll probabaly be fixing it later.
The above paragraphs refer to the door handle only, not the window crank.