the surging at cruising speed can be fixed by changing the jets, I've done it to an old fiat with del-orto carbs once before. We just got a whole heap of different jet sizes and changed them until the problem went away. Or you can, get on a dyno with a AF meter and you will be able to get the jetting perfect.
The problem when you floor it and the motor dies sounds like the accelerator pump is either not working at all, or is working too well and flooding the motor when you push the throttle quickely.
I would be getting the vacuum advance setup first, it may fix some of the 2000-3000 flatspot. The carbie itself may be in need of reconditioning
By far the best way to solve these problems is on a dyno with some who really knows there stuff about carbies. But when they are setup properly it will give the engine more response than an efi engine (in my opinion)
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