"engine was recently rebuilt".
By whom?
If the engine had been rebuilt by me, and if the carbs and ignition timing seem to be more or less in spec, you would suspect the camshaft timing is off.
You would also check compression and then move to a leak down test because who knows what other mistakes I might have made in the rebuild.
I've had a similar problem - undiagnosed misfire under load at high speed - for two years after rebuilding my MGA. I finally suspect a porosity in the water jacket in the engine block as the most likely candidate and have pulled the engine. Replacing with a "refreshed" spare. Hope it runs!
