The tip is not to be able to do 40 bodyweight chins.But to do weighed chins instead.
Once you can do about 15 bodyweight chins.Stop practicing with bodyweight only and put on some weight.Have a dumbbell between your legs and start chinning.
You can do 40 bodyweight chins and yet have a minimum v-shape.You can do 40 reps coz you have muscular endurance but not because
you have the size and strength.
I saw this skinny guy do 30+ chins but he looked like he never touched a weight before in his entire life...
Another tip is to thrash wide grip chins.The notion that wide grip chins=wide lats is rubbish.Wide grip=less range of motion=less muscles fibers stimulated.Furthermore,wide grip is not good on your shoulders(especially for long limb trainees like me)
Do shoulder width grip chins instead.