Motor box after rebuild

My motor box won't fully close after having the engine rebuilt. Any ideas? Video explanation here:

Nice resource. Thanks! It turns out my manifolds and risers are an inch further apart than they should be. But as far as I can tell the new manifolds are the same size and shape as the old ones. So I think it must be that the heads are a different size/shape.

Edit - watched the video and the risers are not different- More information on the heads and other changes would be helpful, there are aftermarket designs with raised exhaust port locations for better exhaust flow. Agree that a good machine shop putting on an angle would be a solid option if they are stock valve covers you would likely have enough clearance to tilt in. I would put on of the old manifolds back on and confirm it also hits the box and maybe have it cut at an angle to test before adjusting the new ones

I experienced this when I installed some aftermarket manifolds on some Edelbrock heads. Not sure which was the culprit, but the motor box wouldn't 'settle' on the floor. Out came my grinder. I 'shaved' a bit off each side of the motor box. Boom. Done. Much cheaper, and the motor box is quite 'thick' in that area.