The carb running lean at a higher RPM can be the carbs fault or actually, not tuned just right to the upper RPM's it's in.
A 750 isn't to large for a stock rebuild, but I would myself gone smaller. A 650.
The myth of to large a carb is true to a point. If you tune it well, it'll work fine. I myself have done a 850 on a otherwise stock 318 equiped with a 360 iron intake. I wasn't to fond of the low RPM (Off idle) feel of it. But it worked until I replaced with a different unit more suited for whatit was doing.
The thing with carbs is the primary bore size can be a bit large for the amount of velocity the venturi is seeing. Fuel metering can be hard to get dialed in just so.
The mix of a large carb, like an 850 and Hwy. gears like 2.76's can be very difficult to get adjusted nicely. This is where a smaller 650 comes in feeling and probably performing better because of the smaller venturis and high velocity the carb see's. The fuel is easier to meter and atomizes better.
Again, any carb can be made to work, how well is another story. Your abiltys will play a big roll. It is easier to go smaller.