First I'd get a carb kit and clean the carb good. The carb kit will give all the settings for the carb, float level (first suspect), air valve spring, choke rod, etc.... If that doesn't correct it then there is another possible fix. You didn't mention how long you were holding it at 3/4 throttle. For this fix to be a problem I would think you would have to be holding 3/4 throttle for some time, like climbing a hill. One of the things that is not paid attention to when installing a V8 is the fuel line. Chevy trucks from the 60s with a six cyclinder have 5/16 steel fuel line from the gas tank, V8s have 3/8 fuel line.
Larry
