There are many wiser people out there, but things I suspect are:
- If the carbs aren't tuned for that engine then they will never work well. You can play with jet sizes and needle heights yourself until it feels OK, but a good carb tuner will make life much easier.
- You may not have enough ignition advance at high revs. What ignition setup have you used? Programmable or vacuum/mechanical advance?
- Is everything else good? Cam timing? Fuel pressure? etc?