Bought a 77 F150 Custom 4x4 and someone changed carbs from a 2 bbl to a 4 bbl. while driving as I come up to speed (50mph) the engine loses power and try's to stall, like I'm running out of gas. Low speed is no problem, do I need to add a electric fuel pump. Engine is a 351M 4speed transmission

 

Have you checked your fuel pressure?