Open Class.. Weight Ideas

[jtslj]

We are holding an Open class race for the Dads this year.. (not put on by the cubs)
only rules are it has to fit on the track.. max width / length of 7" and about 3' wide.
has to use the wood block
and has to use the wheels (the can be altered)
Gravity powered (not sure on this one yet)

what is the ideal weight / placement for this..

I have a car already I can modify with the extra weight/ different wheels I may use.. or I may go with a new design all together..