Bill, if you're building a scratch built frame and asking what size table to start your project upon I can't help with the dimensions for a GT40 build.
However, I did build a frame for a different project some years ago and I can say that it's helpful if it's a metal framed table with a lot of cross bracing under the deck - you want it to be very, very flat and very, very stiff......so your measurements for placement of tubes and hangers and such are spot on. Small differences at the frame tend to extrapolate into larger differences at the wheels and body when they finally go on.
A metal deck is helpful because you can actually weld some supports and jig fixtures right to the deck...which makes holding things steady and in alignment while you're welding tubes and such much easier. Or, alternatively, a magnetic base with an articulating support arm works well too. Metal deck doesn't catch fire either when welding...always good. Just my $.02.