A 38 DGAS is kinda small for a decent breathing 2.4. But when properly tuned it will give very good driveability and torque. The one nice thing about the Weber is that it isSO tuneable, but this very same tuneability can make it a pain to get right, as there are too many parameters which can be easily f****d up!
A Holley 500 will make more power however, I've run many of them on 2.0, 2.2, and 2.4 litre engines. (38 DGAS flows 330 cfm BTW). There are a few things that need to be modified to make the 500 Holley 'right' for any 4-cylinder Opel, the carburetor's idle circuits and power valve circuit are literally twice the size as those on a Weber (it was designed for a V8 after all), so it needs some reworking to retain good driveability. When I get home tonight, I'll look up the specs I used on my friend's 2.5 Opel, it should be *close* to what you require. At least it's a starting point for jets, idle circuit passages, power valve passages, power valve vacuum value, etc.
Bob