The best source for real GT40 body panels is Safir GT40 Spares Ltd In Cinncinnatti (which I have undoubtedly misspelled; I have decided to try to spell it differently each time I write it, it's more fun that way). They built me a body for my GT40 project that was made in John Wyer's molds. It was not cheap, but their quality is first-class and they are great guys to deal with. They have a lot of other parts as well, everything except the tub.
Where to get an authentic GT40 steel tub is sort of the question of the hour. Lee Holman has Tennant Panels tubs, but they are very expensive. Safir are able to supply REPLACEMENT ONLY MkV tubs for cars that are wrecked, etc. No more Mark V cars will be built. This does not preclude your building a Mark II car- if you could find a tub. Many people on the Forum are making their own tubs, which is a formidable project. Jim Dunham is doing this, and from the pictures I have seen is doing a very good job indeed. I recall, I think, that Chris Melia in UK was trying to get a source together for monocoques- whether he has been successful I don't know. ERA build a nice monocoque- whether they would sell you just the tub I don't know.
AND, as if all this weren't bad enough, there are folks who want to replicate a Mark IV car. The tub structure of these was a composite aluminum honeycomb structure heat-bonded together and reinforced with riveted gussets, as I understand it. To duplicate that technology would be almost impossibly expensive and difficult.
HOWEVER, if you were really determined to have a Mark IV tub, there is another way you could do it; (this would get you something similar looking) which would be to build the structure out of preformed FRP structurals covered with FRP sheet (these elements are easy to find and buy), and then spray the entire structure silver. You would end up with something visually similar, strong enough by far, and probably not too much heavier than the aluminum original item. You would have to work out hard points to attach the suspension members and drivetrain mounts. Obviously, Chaparral did this and won races so it is possible.
So there you are. If you find a source for reproduction GT40 Mk I or Mk II tubs, do us all a favor and let us know. You'll be a popular guy....