Well you run a belt drive from the motor back to your tranny or transaxle, both pulleys there are horizontal configuration, just like the stock set up. Transaxle that's all you need. If you run a tranny, 700 or otherwise, input is on a horizontal axis, output however is on a vertical axis and there you go to chain drive down to the sprocket on your live axle.
Chains will not work on a horizontal plane, or at least not for long anyhow! Likewise you need the belt in there to allow it to slip, that little bit of slip is your clutch.