503 does not have a TLS which is exactly my point. The TLS was a bandaid to stop the belts from tearing themselves apart from the torque of the 600 motor. I say bandaid because with a clutch pack they didn't have to go back to the drawing board on their drive system to produce somthing that would live for a decent amount of time.
TLKD put the chain drive on, you won't notice a performance difference, You will be happy you did put it on. If you go with a 503 system, the first time you shock load that thing in a little low spot and blow all the cogs off the belt you'll be pi$$ed. At least try the chain drive, you can allways go back.