How much HP do you want to make? How much money do you have available? Do you have smog regulations?

If you go v8 you will need the biggest brakes you can find. I would also try and get a wider front wheel to control the plowing effect you might experience with a heavy v8.

If you want a cheap v6 that might fit the crossfire you should take a look at the Sebring/Prowler motor and are almost always in junkyards, some are known to have a forged lower end and variable intake manifolds, and have been know to make 300 hp N/A. with the forged lower end you now have the option of boosting too. But you will need to get yourself an aftermarket ECU just to properly tune the motor, though I guess you could get by with the Sebring ECU... The nice thing about the Sebring/prowler motor is that you might be able to sneak it past the smog inspector.