I do not pretend to any expertise on Weber downdraft carbs but can repeat what I was told...IDAs are an earlier design which have a larger float bowl chamber. I recall being advised that a motor equipped with IDAs was better for track use as the fuel reservoirs were larger. IDFs would run smoother on the street and idle better, or so I heard.
The distinction may be academic as Webers are very tuneable carbs and there is a large variety of venturi sizes and emulsion tubes available. IDAs cost more, if you look on Pierce or Inglese's web price lists. Both sell complete kits.
If I were going to drive the car on the street I would get IDFs. The smaller reservoir would mean at least in theory that fuel would turn over faster in the float bowl, and not hang around to get warm for as long. This would hopefully translate into less vapor lock problems. GT40s have a problem with engine room temperatures when run at low speeds. This is one reason why EFI works better, from what I've heard- the fuel gets sent back to the tank where it is cooler.
My Weber setup has three 38DCNVFs set crossways on a plenum, with a throttle arrangement like an old Pontiac tripower. It will be interesting to see what sort of hp numbers it makes.
