There's a lot of smoke and mirrors,
concerning these big American V8's.
GM (and Chrysler I believe) have both now adopted J1349 standard rules for measuring hp because of these somewhat optimistic claims (Horsepower sells).
The most powerful Hemi V8's (which doesn't actually a have hemispherical combustion chamber in the head like the originals as this was found to be more prone to detonation than even a ramp head design. They now use a pent-roof design but still call it a "hemi" -sic.).
Info on old Chrysler engine range here:-
http://en.wikipedia.org/wiki/Chrysler_LA_engine
Newer "Hemi" info here:-
http://en.wikipedia.org/wiki/Chrysler_300
The world's most powerful production V8 here:-
http://www.gizmag.com/go/4286/
Care needs to be taken with U.S. power claims. Previous to 1972 they were using Gross hp ratings. After '72 they changed to Net hp.
You can actually find examples of the same 450+ claimed hp (gross) engines re-rated to less than 350hp (net) post 1973.
The famous 350cid small block Chevrolet engine only broke the (real) 300hp barrier with the second LT1 version in 1992. They dipped as low as 145hp with some versions during the 1970's.
Even now when U.S. aftermarket companies make claims, their dyno figures are anything up to 20% higher than Europe and most of the rest of the planet.
One reason for this is when engine dyno tests are conducted the engines are run without any ancilliaries, no intake filter/system and usually with tubular exhaust manifolds (headers) and open collecters (no mufflers).
Another reason is the most popular U.S. chassis dynos (Dyno-Jet) work on a different principle to measure torque compared to what most of us in the EU, and rest are used to, which are usually of the
Eddie-current dynamometer type.
Only the Mustang Dynamometer (trade name) works on the Eddie-current system which is available over in the U.S.A. and could be safely used as a comparison.
Though even then, correction factors and the widespread mis-use & misunderstanding of this needs to be borne in mind.
p.s. NASCAR engine rules stipulate no more than 358cid.