Hot Rods 9 1/2 clutch

I have a 49-53 ford flathead truck stock rebuild motor. I was told I have to use the eleven inch clutch, is this true? right now I have the 9 1/2 clutch and kills the motor when I let go to engage.

 

That doesn’t seem like a clutch issue to me. Maybe a better tune on the engine is in order?

 

Can you let the clutch out in neutral? Give us more info on the flywheel and pressure plate. You don't want the 11" clutch.