To me the question is not how much horse power, how much TRACTION and whats the right GEARING for the horse power.
If you can't hook up 400, 500 is rather pointless, see my point?
Going with to low of an overall 1st and 2nd gear ratio is just a waste of tires and time. The trans 1st gear ratio need to take into account the rear gear ratio for a reasonable 'hole shot' on 'street' tires. Well, unless your building a dragster only and plan on running slicks.
For maximum enjoyment (street 90% of the time) consider a cam shaft profile that works well with your cruising rpm, which is determined by your final gear ratio.
Say you go with a 5 speed with over drive, cruise rpm is 2200 at 70. Then you have cam with a profile of 3500-4000 and up. You'll find yourself down shifting to keep the motor happy, OR, cruising at 100 mph to get within the cam's profile rpm.
500 horse and ultra low rear gear\trans ratio will result in a wild ride with lots of tire smoke and a tail happy car, that WON'T be 'fast'. With proper gearing and tires 500 or more is quite managable.