Pulsing in 4th gear at crusing speed

[Arl37]

69 C10 307 with 700R4. In 4th at 65 the car pulses like I have the timing set to fast or someone is very light pumping throttled pedal as they cruse. Timing set at 4 degrees BTDC like manual says. Dosent do it in 3 rd. New HEI dist and wires. New Edlebrock Performer 4 barrel. Stock other than headers. Truck runs great except for pulsing. Dont think it did this before I installed new Performer but I cant be sure. This truck is a interior gutted rebuld and just finally started driving to try to get all bugs out of it.