We did not add a lot of weight to our Vega until we put the big block in it.... with 600 odd (sea level) horsepower, we were going 215 to 220 mph with a car that, with the driver in, weighed less than 3000 lbs.
With our lakester (C Gas & +/- 850 hp), the car weighed just under 3000 lbs with the driver in and we felt that the backend was a little "light" after our first successful record setting attempts... (world finals 2010) .... so, we added a sturdy platform, if you will, for the purpose of attaching ballast. We had 400 lbs of ballast weight to add in but we ended up adding only 250 extra pounds (including the 10 extra pounds of attachment hardware) .... it does not sound like a lot really (especially in light of the fact that the Vega got over 2000 extra pounds), but it was or seemed to all that the car needed.
Now, if I was to set our Vega up with a 300 hp (turbo'd G or H class maybe?) engine, I would be removing a lot of weight from the car....
So, IMO, don't build your 300 hp car to be super heavy - although having the ability to add some weight in the rear axle area may be a good idea.
Good luck.