I would recommend you talk to two companies about this before you begin.
http://www.piercemanifolds.com/products.htm
http://www.inglese.com/
Since you are in Australia you could e-mail them.
The cross sectional area of the intake has a very large effect on the RPMs where the engine makes peak torque. There are two simple formulas for calculating this.
The first (and more accurate) is:
Cross Sectional Area of a Port:
Area = Volume of One Cylinder(Cubic Inches) x Peak Torque RPM / 88200
Once you have the Area (in Sq. Inches) you divide it by 3.14 then take the square root of the resulting number and multiply that by 2 to get the diameter of a round port in inches. Then multiply that by 25.4 to get millimeters.
For your car, assuming a torque peak at 5,000 rpm which should give us a power peak at about 6,500 rpm, we get 2.46 sq. in. – 1.77 inches dia. – 44.95 mm.
If your current carbs have 38 mm venturis, working it backwards would give you a cross sectional area of 1.76 sq inches and that would give a torque peak at 3,575 RPM, which is right for a power peak at a little over 5,000 RPM.
If you current carbs have 40 mm venturis, working it backwards would give you a cross sectional area of 1.95 sq inches and that would give a torque peak at 3,950 RPM, which is right for a power peak at about 5,500 RPM.
In either case, you can see that the carbs are sized to make peak torque and power much lower in the RPM range than the Heads and Cam.
The other Formula is actually a “Rule of thumb” that was developed by Alfa Romeo for sizing webers. This is not as accurate as the other formula and only gives you a ballpark approximation. It is:
Size (mm) = CC’s (of 1 cylinder) x Peak HP RPM x Square Root / 50 (for Venturi size)
With this we get 43mm for a 6,500 rpm power peak.
Weber makes 44 mm and 46 mm venturis for the 48 IDAs. The 46 mm are really too large for a 48 mm carb body, don’t idle well and have poor low speed throttle response. The 44 mm venturis will be much better all around. As you can see, you are at the upper limit of Weber 48s with your engine. If you try to go higher at some point in future you will have to go to EFI or a single 4 barrel carb.