It is a good idea to check out the carb, make sure everything is clean, adjusted properly and it has the OEM jets and power valve to establish a baseline. You can look it up using the part # - possibly 4777. BUT.....that is the wrong carb for the application! Double pump carbs tend to run on the rich side for higher performance engines. But if there aren't enough cubic inches, converter, gearing, etc. to to get a good vacuum signal, it's not going to behave properly. Holley's can work well OOTB if they are used on a combo close to their designed application. I would bet that it is a lean bog unless the pump cam and squirters are wayyyyy too big. Either way, the carb won't be easy to "bandaid" tune to the heavy truck.
I also suggest a 600-650 vacuum secondary carb.