500 Hp Slant 6

It can not be done motor only. The head will not flow enough to support that kind of number N/A, and you can't port it enough to get close.

You can get close with nitrous, but it will be a parts eating money pit.

Turbo will be easiest, and at the HP level will also be easiest on parts. Spend the money to do it right, and you can build a 500 HP street motor that will live a long and happy life, and probably with a very basic short block.

Then you just have to upgrade the rest of the drivetrain and the car to handle it.