Chris, you don't seem to be having much luck sourcing a GTD chassis. Having read previous posts from your friend, it appears you are on a budget. I think most on this site would agree that these cars cannot be built to a professional standard without parting with a lot of cash. For example, if you go to a '40 supplier it costs a set amount even for the basic kit. If you want a stainless crossover exhaust, prepare to forfeit going on holiday this year. If you want a stunning paint job for your pride and joy, it will cost a fortune.
What I am getting at is that whatever anyone tells you, it will cost more than you originally estimate. You can however source some of your parts secondhand...this site is a great starting point. These secondhand parts still have a certain value as people know what something is worth. You could shop around or hope you spot the odd bargain at the autojumble but effectively a lot of the GT40's parts are unique and aren't interchangeable with other kits. Screen, seats, fuel filler caps are just a few examples.
What are your engineering skills like? Can you rebuild engines/gearboxes, weld, spray paint, manufacture panels and so on? Have you got a large workshop? Do you own the necessary tools? If not, you will have to get someone else to do these for you. This is where your money will go. If you DO have the necessary skills, and money is tight, I would personally get the measurements for a chassis and make one. If not you're going to pay market value for it all and your costs will spiral out of control. You still need a body after that, and there aren't many people who would let you take a mould off their car, so again your hands are tied. A decent gelcoated body would save a few grand on paint...doesn't Mr Melia build bodies for racing in gelcoat? I know he mentioned a chassis that he'd built and sold.
Not knowing your personal circumstances, I haven't a clue what you are expecting to pay. All I would say is that if you want a job doing properly, do it yourself. Failing that, you'd better know people who own fully tooled workshops and a spray facility who are prepared to let you use these services for free. Good luck with your project!