each spring needs to hold up about 658# per side according to your scale report. So if your car was at the correct ride height meaning 3/4 +- shock ram showing above the bumper with the dimensions you just gave, then the new springs will require a higher setting to maintain the ride height. Go 658 divided by 300 = preload in inches to hold 658# that is 2.19 at ride height not what the platform needs to be. You have left out the spring ride height or strut travel dimension for the calculation. If you want to guess, the platform may need to be 3/4" higher than it was. Remember that you can easily change the spring platform when you lift the car back off it's feet. I would recommend that you try the 3/4" higher or 5.25" at ride height and take a look at where you ended up on your ride height. Unless you are installing this strut on the moon where you don't have an oppertunity to adjust it, you might just need to make one final adjustment to get it perfect and I feel that the work is worth the time.
Sorry I can not give a more pointed answer with the info available, but you should be in good shape from here wont you?
Good luck in 09

There is about 1.5" of shock ram showing when the car is at the ride height with the current 400lbs springs. I could drop the nose a little, but not more than 0.3" or the headers will touch the ground. Do I need to get taller front tires if I want to drop it more with new 300lbs springs or is there another way to get the shock ram showing only 0.75 inch?

Yes you should buy taller front tires, You have used up the travel or the strut at ride height which will prevent pitch rotation like a solid suspension does. I like to run the strut on the bumper on the cars I build, but the 3/4" figure is what Rick Jones and others recomend for ride height set up. You may even be able to use a 275# spring to aide in weight transfer if the spring will hold up the car which I am sure it would based on the last info you reported.
I just had this notion to ask you, The engine weight is in the car when you are checking this isn't it?

It depends on how much total travel the strut has. Most racing struts only have a total of 1 1/2" of travel from fully extended to the snubber. If that's the case with your struts with 1 1/2" showing now your strut is "topped out" or close to it. It might be that someone has tried to raise the car from the ride height it was built for which is a difficult thing to do with a strut car. If there is no extension left in the struts with the car at it's desired ride height you will need to either lower the car, use a bigger tire or change the strut mount.

I would recommend using the hypercoil spring. They're a much better spring than the springs you linked to. They're a little more expensive but worth it. You can buy them from strange. I noticed the springs you linked to are priced each so double the price they have there.

Yes, the weights include everything. Car race ready, fueled full and me sitting in the car.