Before you can start drilling the forward top fuselage to the fuselage, you have to make a very slight bend in the part of the firewall above the top horizontal angle.
So, the bend has at the same height of the angle which is on the inside.
The firewall is flimsy material and you have to take care how you do this. You can't just push it over.

I found a piece of stock angle long enough and clamped it on the forward side of the firewall at the same height of the angle on the inside.

Then you just bend it over by hand. Carefully and little at a time. I overbend it a bit so the images below are actually exagerated. I think the angle has to be 8 degrees but it's not really documented somewhere how much it should be.