392 Hemi & firewall

What is on the back of the 392 Hemi that would interfere with the firewall? Are all the exterior dimensions the same between the 5.7/6.1/6.4? I have been looking through all of the Hemi build threads looking for 392 build specifically. I had seen someone mention the 392 interfered with the firewall. But couldn't find specifics. Is it actually the engine or is it the transmission that hits the firewall?