440 power gains/ aluminum head advantages?

I had another thought…….
If the OP has a friend with something like a “known good” 850dp carb he could borrow and bolt on, that might be worth a try.
It’s basically just a tool/process to rule out any possible issues with the OP’s current carb.

I feel like there has to be some component that’s way off.
It seems unbelievable you could have that combo of parts run that far below expectations without there being a real “problem” with……….something.

To say it another way…….
IMO, the sub-par performance isn’t the result of how good or bad that exact combination of parts works together.
The poor performance is the result of there being something wrong with how at least one of the items in the combo is working/functioning.