I think if it were me, I'd at least try a simple test to help confirm it's the carbs.
Try advancing the timing about five degrees from where it is now and go for a short, gentle drive (don't rev it very high). Is there any change at all in the flat spot?
If not, the carbs are probably the guilty parties. But if the flat spot does change at all, for better or worse, maybe look into tweaking or replacing the dizzy before messing with the carbs.
After the test, you'll need to set the timing back to where it was (assuming it was right to begin with).
This is not a definitive test, but it may give you another data point, and before investing that kind of coin, I'd want all the data I could get.