I purchased and fitted a Langs 3963C clamp bolt flanged fan pulley but now the crank only just engages with the roll pin.
I purchased a new crank dog clutch but it is worse.
Has anyone had experience of this, what do I do?
I have same issue as you. Didn't discover it until I put the engine back in the frame. I've had no problem starting my engine so for now I'm going to leave it alone.
It appears that the depth of the i.d. bore face is insufficient to fully engage the crank pawl. Eventually I plan to remove the pulley and face off the bottom to get the clearance needed.
The new pulley isn't made correctly. Call Lang's and see if they have updated stock for a replacement. You can see by their image that the roll pin hole is drilled along the bottom.
The original is cupped to allow clearance of the crank dog.