Q: WHEN I ACCELERTE FROM A STOP ITS FINE IT LEAVES BUT ONCE IM GOING 45 MPH IT BOGGS DOWN AND WONT ACCELARATE

I have an old classic pick up 1952 International. L110. It has a 350 Pontiac engine 4.9 with a carter 4bbl carb. I drive only for local cruises. I can accelerate fine but once I have going approx 45-50 mPH I try to step on gas to keep momentum and engine boggs down. I can then also slam on gas to open all 4bbl and same thing. only RPM sound higher but the vehicle wont move. I have tried moving the fuel mixture screws and it helps a little but I still dont get all the HP that the engine can do. any ideas?
My car has an automatic transmission.