Martin The uprights are some of the cheaper parts of all the suspension parts. From Cobra restorers the rear uprights are $650.00 each and the fronts are $275.00 each. All the other parts add up quickly. For the front you need the stub axle and the upper clevis. The front and rear hubs are a major expensive part roughly $2000.00. The original style brakes will set you back a pile of cash as will the correct center section for the rear.
A jag can be made to work with a factory original style frame if the correct mounts are made. The Jag and Cobra are close in shape . The Jag does not have the front mounting bosses that the Cobra has but can still be used.
If you look here
scratch build you can see how close of a fit it is and why a top mounted Jag will not work.
This is the Scratch Builders Forum and here there is a large range of scratch builds. They range from purchasing all the parts and paying someone else to assemble and paint there car. The other end of the spectrum is where they build everything themselves.
You could build your chassis to original specs and use other donor components and when you can afford or locate the correct parts unbolt and then install the correct parts.
If you go this route it will keep you out of the bars for awhile.
Mark