While the 440 could most probably & happily use the 750, there isn’t a whole lot of power gain or rpm gain. The one reason I would move to the 750 is if he upgraded the engine.
History - An old friend of mine had a ‘66 Imperial w/a 440. The carb was a small diameter inlet AFB that also had no secondary air door above the throttle blades.
This carb is a 600 cfm unit. It never bogged or stumbled. Even when you slammed the go pedal. The secondary side would open up and just gobble down the air and fuel and the car would just GO!
If your bud wants a 90+% cruiser, I’d use the 600/650.
