Mounting mecanum wheels... please help?

[Governators3361]

This is our first year using mecanum wheels; we have no experience with them. Furthermore, just before the build season, our only engineering coach found out that she pretty much wouldn't be available at any point, so we are short on professional advice.

The attached image is the general frame of our drivetrain, but is definitely subject to change, and is really all the metal we have to use for the drivetrain. It is 32 inches long by 25 inches wide, and this is not changing unless it absolutely has to. We are using the Cim motors that came in the kit of parts and we have enough AndyMark gearboxes to put one on each wheel. Our issue with this is the amount of room available in which to put gearboxes.

The mecanum wheels are 6 inches in diameter and 1.8 inches wide. Right now we are planning to have them roughly 20 inches apart. We would greatly appreciate suggestions on how to mount the wheels and the gearboxes, and if there is an ideal alternative to gearboxes, how to best reduce the RPM from motor to wheel, including specifics of gear size and distance between them. Thanks in advance for any help you are willing to give!