Hi Laurent,
There are three different issues being addressed here: 1) Driveshaft angles 2) Nose down motor 3) Carburetor angle. Some actions solve one or more issues or can create a new one...
The drivetrain (engine and transaxle assembly) was parallel to the chassis in the original GT40. Fortunately you have a transaxle that can be inverted so the nose down drivetrain situation will not effect your car, plus brings the driveshafts up. It's worth the trouble (cost) to flip the diff and have the drivetrain level.
Installing the motor .5 above parallel to reduce air pockets forming in the cylinder heads can also save you having to run steam passages from the back of the intake manifold to the front (there's a few threads on this somewhere).
In all cases a good header tank with bubble bleeds from the radiator and the top of the intake manifold is highly recommended IMHO.
The carburetor wedge plate is an excellent method of bringing the carburetor level with the engine. The angle in question relates to the original application of the manifold on Ford production vehicles. Truck engines were more inclined than passenger cars and as such had greater carb mounting pad angle.
The Cobra and GT40 manifolds for Weber carbs have no angle built in and several four barrel manifolds are available with no carb angle.
The final choice of what to do about carb angle comes when you know the final choice of induction system. This could change as the build progresses.
The best use of the intake manifold at this time is to remove the carb and bolt a lifting plate in it's place for your engine crane. The drivetrain will be in and out a few times...
The nice thing about sorting the position of the drivetrain now is that all the other plumbing, wiring, linkages and shifter mechanisms etc. can be planned, test fitted and prepped for final assembly.
The great thing about this forum is the wealth of information available by searching threads or just asking. Lot's of us will be watching your car build and ready to chirp in anytime you like
Cheers
Ian