Right - apparently there is a 6.4L! It was unveiled at SEMA late in 2005.
thehemi.com:
http://www.thehemi.com/news.php?id=20060228-1
From forum elsewhere:
"Well I read in last months Hot Rod Mag that the 6.4L already makes more HP/Liter then the new Z06 motor(LS7). Hot Rod just ordered a crate 6.4L for some dyno testing."
Allpar.com:
"The 6.4 liter version, at a historic 392 cubic inches, due in 2008 with the Dodge Challenger, does the SRT-8 one better, and we'd like to know how since the extra size isn't all that much. It generates a full 505 horsepower. Because it’s a crate engine for racing only, it’s quite possible that some sacrifices made for gas mileage and emissions reduction in even the SRT-8 version (6.1 liters, created in mid-2005, and not using multiple diplacement) have been removed. However, this is a race-only engine, and is not emissions legal or necessarily even practical for street use."
automobile.com:
"But when it comes to market, DCX might up the ante in SRT8 form by dropping in its latest HEMI creation, a 510+hp 6.4-liter (392 cu-in) HEMI V8 which was recently displayed at the SEMA motor show in a 1970 Challenger R/T Convertible, no less. These two combinations show that DCX is in tune with North American car enthusiasts, and as usual will most likely commit the Challenger concept, 6.4-liter HEMI upgrade or not, to production."