Well it all boils down to this...
All cars have a gas pedal, so if you have 650hp, you don't have to use all of it.
As for what parts go into the engine, a lot of guys that do their own work (and get their advice from various forums) tend to go overboard on cam selection and cylinder head port volume size.
As everyone knows, the larger displacement of the engine, the less radical it has to be to make more horsepower. So as a baseline, if you start with a 427ci engine, you're automatically in a position to make more horsepower with less effort/issues. For example, it's almost downright simple to make 500hp with a 427-428 cubic inch engine. It doesn't take a real rowdy cam, high peak rpms, large port volumes that cause low rpm sluggishness, etc. At the other extreme, getting 500hp from a 289ci engine takes the exact opposite: high static compression ratios, large cams (rpms make power), large intakes that have huge plenum volumes, large port volumes on heads, etc.
With a Cobra, you can get away with a little more "rowdiness" just because of the light weight of the cars and most cars have manual transmissions.
If some of you would be interested in seeing a list of different engine displacements, "streetable horsepower" associated with each category, and typically what parts it takes to get there, I could put something like that together.