A good basic rule is you want the bottom of the pan (the area your butt to shoulders rides on) to be about the same height as the axles. This is more for handling than anything. If your luge has too much flex you'll know it as it'll scrap when you roll over bumps (this is not good). You can easily set the ride height by placing risers under your trucks. Khiro makes great risers. You can have your choice of hard plastic or rubber risers. The rubber risers asborbs some of the road vibrations.
Place your front truck as close to under your butt as possible and the rear truck under your head. This will get you close to a 60-40 split in weight distribution.
Overall design wise avoid sharp edges or pinch points. If you design it with the idea that you're riding it at 50 mph and at some point the luge comes to a complete stop and you don't (your body will slide rapidly forward off the luge) you don't anything in the design that will tear you up, trap a body part or cut you.
Clearly you've got design skills and can illustrate your ideas well. Sounds like you have manufacturing skils (welding, cutting, etc). The idea of a plasic pan doesn't sit well with me. I believe that you will not get the ridgitiy out of the plastic that you'll need. Just my thinking. I look forward to seeing more of your ideas and the project as it progresses. Have fun with it.