Not sure, I think around 8 inches as you can see by the cowl. It had bucket seats and a console between like your Jeep. The console was 10 inches wise so lots of room. Might have only been 6 inches, I think the radiator has been widened less than the body.
This is the only photo can find that matches, Amelia Airhart had this exact same coupe version, the cowl looks wide anyway.
I would suggest less is better, I saw a 32 coupe widened and it looked dreadful.