They ran no pressure regulators at all and full 7 psi to the carbs. I presume the Shelby GT40's did also, but can't verify that?
The only one I saw up close was 103 (one of the three original Team Shelby cars) when Bill Wonder owned the car in the '70s. It had Gurney Westlake heads and 48 IDA's on it then but that was before my Weber life began so I didn't notice details like that.
I do know that the Shelby factory comp cars all ran SAE 3/8" id fuel hose to the Webers and for that there were special SAE banjo fittings made, not the metric fittings that come with the carbs now.
Those DO HELP the fuel volume FLOW to the carbs.
The biggest issue with stock Webbers is that the inlet valve and seat, original ones, are crap.
On IDA's I have run I found it necessary to run the "glass ball" high pressure valves to keep them from peculating fuel immediately after shutting off a hot engine.
I believe the only source of them is Gene Berg, and he calls them "high pressure" needle and seat. About $100 for four as I recall, but necessary. If you are running IDA's and you are using the Original Weber supplied needle and seat, change them over now before you have a barn fire.
You haven't seen anything in your life until you see your car with fuel coming out of the carbs and running all over the hot engine. Your life will flash before your eyes and I can tell you that the adrenalin rush at that instant freakin' hurts. like you got hit with an electrical shock. A big one.
Using the high pressure valves, really opens the door to running more fuel pressure but if you run dual pumps, i.e., an electric pressurizing a mechanical pump and full 3/8 lines to the carbs you are supply the volume the carbs need to feed the engine.
You should run a pressure regulator on the street. The one to run is the Holley red and set it a 4 psi. Unless you are going to run against the big factory teams at Lemans, that is all you need.
Even tuning Webers on an engine dyno at maximum rpm is a pretty hairy situation. I don't really think that is necessary, at least to me it isn't.
It isn't a bad idea either to replace the original brass floats with nitrofil ones. The amount of pressure the brass floats can stand is questionable.
I haven't collapsed any IDA floats yet, but I have on around a dozen or so Holley carbs.
You can't collapse the nitrofil and is the only safe thing to do.
It's also unpredictable what the E10 pump gas we use in the US will do to the brass floats. The alcohol content was deemed "safe" by someone, probably a vote in Congress and it probably won by one vote, so you should know what that means...I wouldn't bet my life on it? You're a big boy. You can make your own choices.