[asid61]

Off the shelf is great, but I think it's better to optimize for your application. For example, the vex ballshifters can be used as winched for hundreds of pounds of springs- not something you would see in a drivetrain usually. GRT maximized their ballshifter's potential by using smaller shafts, bearings, etc. to make a smaller and lighter gearbox than what was on the market from any of the normal suppliers.

So looking at your cad file (please, export as .stp so I can see lines!) I am noticing a lack of detail. Being as this is your first chassis, why not put everything in? My first gearbox was horribly screwed up because I didn't put all the components in the cad such as screws and nuts and led to a lot of problems down the road.
Download the cad files of all stock components (McMaster has cad for their screws and other products if your computer is decent) and keep a detailed parts list that is organized in some way. This includes wheels, screws, nuts, shafts(including e-clip or circlip slots), spacers, bearings, etc. Electronics are also a major plus even if the whole chassis will be open for them.
This produces a wicked looking cad and hopefully something that requires minimal effort to put together for testing. It takes less time in cad than in real life to change dimensions.

Side note, what cad software do you use?