Have you checked for a blown PV? You really need to rule that out. Your jetting doesn't sound that fat for the carb, although a 750DP isn't ideal for your car. A reduction in jetting might help the problem but I think its a partial bandaid for whatever else might be wrong. Might you have 50cc accelerator pumps on it? (instead of 30's, at least on the primary)
Do you drive with a reasonably steady foot?--DP's can kill MPG if the driver is a constant pedal modulator!!!
Thats a pretty big cam if you don't have say at least 9:1 compression to match.
Is your 4-6 mpg number accurate or might it really be 6-8? Is your odometer accurate or does it read 'slow'?

just random thoughts