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First off, forget about building two. A Cobra is going to cost a lot of money, and if you had a lot of money, you wouldn't be asking these questions. Buy or find a nice kit (there are plenty out there) and take some time to build your own Cobra. Realize, though, that you will spend more building your own than buying a nice used one. Every option under the sun must be considered, but you always can save by using a (somewhat) stock Ford engine, at least for the short term. As for welding up your own, I say leave that to the pros, it's what they do for a living and you get a Certificate of Origin, which will be required to register it when it's done. Lots of info here on kits, so do some homework and visit as many manufacturers as you can. I would (I did) try to buy one somewhat close to home, to keep shipping and visiting costs down. Good luck.