For the 2105 season, Team 1108's robot Tote-O used a stock andymark kit frame and four stock kit Toughbox Mini gear boxes to run eight-inch mecanum wheels.
The front wheels were run on dead axles with sprockets to transmissions that were mounted in the stock location at the center of the frame rail. The sprockets need to have same tooth-number sprocket as the axle.
The back wheels were mounted directly on toughbox minis' shafts, which had been extended (we welded, but you can buy these too). To mount the backs, we had to create some careful holes in the frame, but it wasn't too bad. Just laid it out 1:1 in paper, glued it to the rails, then cut the holes with a rotozip on a dremel.
We also had to cut all new churros so we'd have enough space between the inner and outer frame rails.
Here's the blue-alliance photo of it, and a photo from our facebook album.
blue alliance:
https://www.thebluealliance.com/team/1108/2015
facebook:
https://www.facebook.com/13074222031...type=3&theater
I still have CAD of this if you think you'd need it. It wasn't too hard to do.
Good luck.