392 in a 70 Duster

I dont think the 6.4 will let you drop the body down over it without firewall work. The 6.1 has more clearance in the back. It's a little tricky with the alternator by the frame rail but you can just lower the car over the engine once you clearance your firewall.
You might take your intake off and lower the car over everything, then you will see where you need to do work.