In black the drawing of the Leopard 2 prototype with 120 mm smoothbore gun from page 257 of Waffensysteme Leopard 1 und Leopard 2, with in blue the Leopard 1A3 turret (minus some of the sticking-out bits) from page 78 of that same book:
They’re not to the same scale in the book, but because of the dimensions I could easily scale the 1A3 down slightly to match the 2. I then lined up the turret centres. For the side view, I also lined up the roofs, as the underside is just about impossible because on the 1A3 there’s stuff in front of the lower edge, making it hard to see where that is, exactly.
As I suspected, changing a 1A3 or 1A4 kit turret will be a ton of work because just about nothing is the exact same size, shape or position.