Update:
Eight months later I have installed a wideband O2 sensor and significantly improved the jetting on the weber 38/38. Mixture is now always somewhere between 13.5:1 and 16:1, with a steady 13.5 during WOT all the way to the shift point, which is now around 4800 rpm. I'm going to make one more round on the jetting, but it's getting complicated now as I think I've nailed the mains and idles and will have to turn my attention to the emulsion tubes, which are a bit mysterious to me.
Power is much much better, and I picked up 600 rpm on the power band as the car used to fall on its face at 4200 and now does so at 4800. The stock cam is the big limitation here, and I can safely say money would have been better spent on a cam than on my fancy-pants rocker assembly. Still, I'm glad I have them. Highway cruising is awesome. With my 3.25 gearing 60 MPH is quite relaxed, and there is brisk, albeit clamorous acceleration up to at least 90 mph. That's as fast as I've wanted to go.
In summary, the headwork made a huge difference and truly brought out the utility in having headers and the bigger carb. That said, without a much hotter cam, you cannot expect to tap but a fraction of the potential of this very expensive modifiaction. Nevertheless, I can now see how one of these engines, especially when mated to a five-speed (mines a c-4), could actually make for a fun sportscar engine. Also very important: It is worth every penny to own a wideband O2 sensor!!! I cannot tell you how far off my jetting was, and has been for the last four years.