I know you've seen it already, but just look a little harder. All you need to do is offer up the 3905 tranny to the big hole in your revo chassis, and it should be immediately clear where you need to remove material to allow the tranny to fit flush with the chassis. Then, make a cardboard (cereal box is best) template that allows you to mount the tranny on the side that needs extra chassis material. You should also be able to see where you need to drill holes to mount the tranny on either side.
Print out that picture and the others to use for reference (they didnt have any help so it really isnt that tricky, just needs a little imagination- remove a small amount of material each time, rather than all at once....).