more response for DAD (I like your avatar by the way)

the advertised power #'s are "engine" baseline and are very consistant
they are based off of a 650 carburetor with a non oxygenated gasoline and a very basic header

common racing fuels and upgraded components can bump them numbers 10 to 12 percent but that is about the top

question of cost difference 602 VS 604
the base engine cost is just over $2200 more for the 604
the main component difference is the ignition
the 602 engine comes from GM with an HEI distributor and keeping with the theme of the entry level "budget" package, we have utilized that distributor.
the 604 engine come with NO distributor from GM so we elected to use a magneto to stay with traditional sprint car accessories eliminating any need for a battery

other ignition forms are applicable for either engine but this is what we decided on for our package engines.

hope that helps a bit

as ever
Brad