Just a suggestion and maybe it is what you have planned, but I would start with it flat or even at a slight down slope and have it adjustable. If you don't need the down force there why create more frontal area and drag with it up? For us I feel the real advantage of a spoiler is it is a legal place to put two fins (the spill plates on the ends) to help with the center of pressure. With the shape of that car you are going to have plenty of center of pressure help and maybe the spoiler is totally unneeded. One other suggestion is that you might want to design the spoiler where if the car does get backwards that it is held on in such a way that it might want to lift up so that it isn't catching so much air when the car is in reverse and might want to fly.
As Porkpie likes to say it isn't so much how you go into the wind, but how you go out of it and that car, sorry, isn't going to go out of it very well at all. I can't remember why you really want to use it, but the worst mistake any of us can do is use a car or motor just because we have it. You are going to put a ton of time and money into that car and at the moment might feel that you just want to see how fast "it" will go, but believe me you will be bitten by wanting to be competitive and then you will be starting all over and it will be frustrating, at least it would be for me, that you didn't start with something more competitive. We are kind of in that place with Hooley's Stude.
Whatever you decide good luck and have fun,
Sum
