My '47 has done that a few times. As mentioned, I start it in gear with the clutch pedal depressed and the other foot on the brake. That's always worked so far. I also block the pedal down with a 2 x 4 when it's going to be sitting for a while.
With one foot on the clutch and one on the brake, how do you depress the starter button , or is that where the 2x4 comes in?