So where is the 6.4L engine documented as a 391?
I have only seen it written as 392, for example...
6.4
Chrysler displayed a larger 6.4 L (392 cu in) Hemi in 2005 with output 525 horsepower (391 kW) and 510 lb·ft (691 N·m) torque. It is based on an iron HEMI 6.1 L engine block with aluminium alloy pistons.
[6] In late 2009 Chrysler Group LLC announced that the 6.4 Hemi would be available in the next generation SRT 8 Dodge Charger, Chrysler 300c and Jeep Grand Cherokee.[
citation needed]
The production version of the 6.4L Hemi will first debut in the 2011 Dodge Challenger SRT8 and it will utilize Variable Camshaft Timing as well as MDS in automatic equipped cars and non-MDS in 6-speed manual cars. Although 2011 Dodge Challenger has not debuted yet, specification of 2011 Dodge Challenger leaked to public.
The SRT8 version will have 392 HEMI (6.4L) engine rated at 475 hp (354 kW) and 460 lb·ft (624 N·m).[7]
The engine will then be used in the Dodge Challenger SRT8, Chrysler 300SRT8 and the Jeep Grand Cherokee SRT8.
A crate engine version was sold under the name '392 Hemi Crate Engine'.
OR
The 6.4 Hemi (392 Hemi) is to be in the 2011 Dodge Challenger SRT8 and other SRT V8 models. The engine is very different from the one in the Challenger Drag Pack (the Mopar crate engine), not just because it has multiple displacement (four cylinders automatically shut off when not needed to dramatically increase gas mileage). The engines have a different bore, stroke, cam, heads, compression, and intake.

oh2o told Allpar readers about the engine in 2005, with horsepower estimated between 450 and 500; in 2008 we firmed our estimate at 480, and in 2010 it was launched with 475 horsepower and 460 lb-ft of torque (on
Challenger; on the
2012 Charger SRT8, it was 465 hp and 465 lb-ft). To limit emissions, both spark plugs were used at top dead center and one was used before top dead center to burn off the hydrocarbons, allowing the use of less restrictive catalytic converters. The engine reportedly (“carcharger”) cannot pass durability testing at 525 horsepower, due to piston skirt issues when tested near overheat conditions. (Durability testing reportedly requires 200,000 miles MTBF and extensive testing under extreme conditions.)
The 392 Hemi (6.4 liter) will not have MDS (four cylinder mode) when paired with a manual transmission.
Or
Will the 2011 Dodge Challenger SRT8 Be Available With the 6.4L HEMI V8?
0

It has been reported for some time that the current 6.1L HEMI V8 would need to be replaced at some point in Dodge performance vehicles. While the 6.1L is a very capable engine with good horsepower and torque characteristics, the landscape in the world of modern muscle cars is evolving and Dodge must evolve with it. With the competition stepping up their horsepower figures to increase sales and rule the streets, Dodge must keep pace or be left behind.
What most are speculating will be the replacement for the current 6.1L will be the new 6.4L HEMI (aka 392 cubic inch HEMI). Current theory is that it is slated for launch on the 2011 Dodge Challenger SRT8 in November 2010. No horsepower figures have been released yet for the production version, but the current 6.4L HEMI V8 crate motor available from Mopar Performance generates a solid 525 bhp and 510 ft-lbs. of torque so that should give us a good idea of what this new power plant is capable of right out of the box.