I think the OP is using the same Holley mounts? But good point either way. Anybody else reading this could get the idea that the motor is in the stock location and this would be bad info for them.
That shot from the top looking down on the steering box makes it pretty obvious that these headers almost certainly won't work without the Holley mounts. The motor is about 1.75" forward of stock with the Holley mounts and that picture makes me think you would have to be at least 1.25" forward to clear the box. But the flange to firewall clearance makes me wonder if it could even move back that much.
It's definitely a system and works well together but probably not so much with other manufacturer parts.