Fuel Starvation?

My 73 Duster was originally a 6 cyl car but now has a 360 mated to a 727. I believe the trans to be mostly stock but the engine has the Edelbrock performer carb (1406), cam and intake. It also has roller rockers and ceramic coated headers along with the J heads. I just switched out the 2.93 gears for some 4.10's and when standing on it in Drive I notice it start to lay down in 3rd gear around 4500 rpm. It doesn't feel like it's out of gas, but it stops pulling and you can feel it just sort of lay there. It will pull to 5500 in first if I shift it but then it stumbles by the time I shift to third and the RPM's start to climb. The fuel pump is upgraded from stock but I don't know the specs are on it, however, it is pulling from the tank thru the stock hard lines which I believe are 5/16". Would I benefit from running 3/8" line all the way to the fuel pump from the tank?