pic: First Sheet Metal Drivetrain CAD
 

Re: pic: First Sheet Metal Drivetrain CAD
 

Looks like my original comment was eaten, trying again.

Not bad. From my experience, I'd recommend making the following changes. Sorry if I come across a bit terse.

Move the flange from the sides of the belly pan to the rail, and make it the full length. That will make your frame stronger. I try my hardest to not break critical flanges like that to keep the strength up.

Make your frame as tall as you can get away with. Again, it will be stronger.

Put another bend on the inside of the front and back rails to make them stronger. I'd also make them at least 1.5", if not 2" deep. Our 2012 chassis has bent front and back rails from impacts, and is beefier than yours.

Lighten your rails more so you can see the belts/chain and get your hands in to fix it.

Not enough rivets on the corners. They will sheer in my experience.

How are you going to replace a belt when it breaks? Between matches?

Re: pic: First Sheet Metal Drivetrain CAD
 

Here's version 2 that incorporates AustinSchuh's advice and adds other improvements including:
-Easily removable gearbox connected via gear to the axle (like 971's recent robots). Increases belly pan space and simplifies belt mounting.
-Outer wheel bearings are in sliding blocks tensioned with cams, allowing for better belt performance.
-WCD-style pulley mounting allows for belt changes without removing wheels.
-1.8lbs lighter

Closeup of cam and bearing block. There's an identical block and cam on the outside. Anyone see problems with this setup?

Re: pic: First Sheet Metal Drivetrain CAD
 

The gearbox as you have it looks pretty difficult to manufacture. Extremely deep and narrow U-channels like that are hard to make with a press brake without some specially designed tooling.

Or is that two pieces? It's a little hard to tell.

Either way, I'd suggest using a flat panel that bolts to the outside of the chassis rail instead. Laser cut bolt holes hold much better tolerances than bends, and you have to keep things pretty tight when center-center gear distances are concerned.

Re: pic: First Sheet Metal Drivetrain CAD
 

As per Joe G.'s advice, the gearbox is now two plates. The lightening pattern is improved and an access hole for the outer tensioners is added.

Re: pic: First Sheet Metal Drivetrain CAD
 

What tolerances can your shop hold on bends? I'm not sure your gearbox is going to mesh with your wheel correctly. You have the bend tolerance of the frame + the bend tolerance of the gearbox. For our shops, I would realistically say that that is +- 20 thou. For a gearbox, you want to hold +- 1 thou on c-c. 971 does this with a dowel pin and a tight tolerance slot in the gearbox plates. That makes it so the laser ends up holding all the tolerances, giving us essentially +- 1 thou on the c-c for the wheels.

I've found it really nice to have a square cutout above our wheels, and cutouts in the frame big enough for hands. That makes it really easy to see how your wheels are wearing, and to get your hands inside if you need to do something.