It depends how good you are. Boosted is great for high-wind motors, and not so good when trying to run against Modified motors.
If you think you can make the top 30 at a National, you need to be looking for a 5.5. If not, then I would suggest a 7.5 or 6.5, which you would need to gear about 46mm/rev using an 88T spur, about 44mm diameter rears and whatever is a suitable pinion. Use no boost on the speedo, and start from there to find the best ratio. Don't think about the motor temps, just focus on gearing to get a fast lap. 12th cars are so light that they won't usually bother the temp gauge in Modified.
If there's anything I've missed for you, please ask. HTH
