From asking around my house hold on the hemi builds the conclusion was to purchase a longer stroke crank shaft. By using a bigger cubic inch motor you would only have to produce around 1.2 HP per cubic inch compared to 1.45 HP per cubic inch with a 426 cubes.This is a camparision of 426 CI to 500 CI. By making the motor even bigger this number could drop down near 1.1 HP per Cubic inch. One Hemi motor in the house hold is much like you have suggested 426 CI with a 30 over and a mild 580 solid lifter came shaft. It needed a lot more compression to produce the Power over 650 HP and also has to be spun much higher in the RPM range to produce this. Also suggested was to go with different Heads than stock cast irons heads if you plan on making the motor bigger. Stage V was the choice here and if you go that direction then porting is recommended and then performance intakes are the next step. If the car is NOT some OEM Gold trailer queen ,build a Hemi Motor that can have changes added to it for more horse power in the future. Stock Cast Iron sometimes limits this with new Technology in the future from Aftermarket sources.Also with bigger cubic inch Hemis it takes less compression to produce your wanted HP motor making the motor more livable on the street with gas out of the Pump.