elliptical radius on an oval

Hello all, I am wanting to build a 8'x16' oval with elliptical radius;s at each end im not sure on how to go about this. I've seen jay birds oval track and that is what i want for my track, so what im asking is what type of jig and what are the steps to go about achieving this. I have built a slotcar track that is routed before so i have some experience in track building, any advice would be helpful. thanks Tim