[68gt500]

Before you even think about changing jets, you have to make sure that the synchronisation of the carbs is spot on. Not only during idle but very importantly also at the higher revs.

If you really want to tune your Webers do yourself a favor an get a WBO2 Gauge, it makes diagnostic so much easier and accurate. Innovate makes a nice unit that does not cost an arm & leg.

That will tell you exactly which circit to enrich or faten up to get the results you want.

Don´t let anybody tell yout that you have to put up with rotten driveability or crazy fuel consumption - the Webers can be tuned to deliver power, driveability and mileage.

There is an italian guy on ebay (alfa1750 or so) that has really great prices on original Weber jets.

The Idle holders are hard to get - I did solder mine shut and drilled new holes. I bought an nice set of carb reamers at a motorcycle shop - makes for much more precise holes than just drilling.

With 400+ HP I would think that you could use the 40 mm venturis - you will love (or be scared of) the new found top end.