Classic symptoms pf fuel pump failure. Wil rev and run fine but under heavy load the engine fail down and will respond only for a short busrt pf power.
If it always runs like crap in third most likely jetted way to lean. Also possible vacum advance is way off as well. But not as lkely as fuel.
Is the edelbrock electric choke from the factory or manual choke. Does this motor have eletric choke and the old motor had manual. Electric choke carbs from edelbrock are fuel milaged tuned manual chokes are performance tuned.
Do you still have the 02 sensor bungs installed. You may want to get a ,ean rich guage on the engine to see whats wrong when it breaks up. If it stay the same it may be vacum advance or leaks.
Hope this helps.