[David]

I always put the weight in the back since potential energy = mass * gravity * height. Mass is limited, gravity is constant, so only height can be maximized. Putting the weight in the back allows the weight to fall the farthest. This assumes the track flattens out before the finish line.

Some tips:
Sharpen the wheel hubs where they contact the body.
Put the axles in a drill press, file off the ridges and make the backside of the head cone shaped so less of the head contacts the wheel. Then polish to mirror finish.
Make it a three wheeler, with one front wheel off the ground.
Don't use the stock axle locations (if allowed), put the wheels at the far ends of the car so it'll track straighter. I flip the block over so the original axle slots are cut off.
And most important, make sure it rolls straight when it's not on the rail.

Our local district has classes for kids and adults, so if dad's wanted to go all out they could without competing against kids.