Smallest spur in B4?

[Danosborne6661]

Basically I'd quite like to try running a smaller spur to stop the motor hanging so far back.
This should make a big difference in handling! But I just wondered how small a spur you can go?

Obviously by having a smaller spur this brings the motor forward but how small can you go before the motor touches the casing? I've seen that AE make a 72t and a 75t which are quite a lot smaller than stock! Would I still be able to gear my car sensible for indoor racing and still get clearance on the rear diff casing?

Just wondered what a sensible size spur would be to start off with and to see what you lot are running