Don't even bother measuring it, it physically won't fit between the original fenderwells, it will need major modifications.
Believe it or not but it will fit with only modification to the pass inner fender/ shock tower, a little str column work, and moving over the master cyl. Check out the link I posted.
I'm not sure if you tried to mock anything up yet, but I did have a 440 sitting in my car, and with a 440, there is only 1" of space between the exhaust ports on the heads and the shock towers. a 426 Hemi will require the complete shock tower assemblys (both sides) and the inner fender that surrounds it to be moved, not to mention the steering box will also have to be relocated because the top of the gear box will not clear the Hemi head in it's stock location. And once that is done, you still have to find a way to route headers, which there is barely room for with a smallblock.
I would say that is major modification, can it be done? yes! but it isn't as easy as some make it sound, especially if it is being done correctly and not butchered.
I know Petty had one in his Cuda, but everyone that I talked to about that car said it was so butchered together that it was unsafe, and everyone knows that he did loose control and go into the stands and killed a kid with it. Not sure how true, but rumor has it that Petty buried the car behind his shop shortly after that crash.
