Snap Krackle Pop On Freeway?

[Amigo Arts]

Hi Everyone! I have been gone for a long while but I guess that is what life does to you, house, two dogs, baby, and started a business.Well here is my question. I have a 71 Chevy long bed. I have dubbed it my shop truck. Its kinda hot rodded too.
So here is my problem. It runs fine in the city 25-45mph. When I get on the freeway all is fine too, sometimes. When cruising at around 55, sometimes it starts to krackle and backfire. Totally random though. I can drive 60 miles and only have it happen once or twice, or sometimes it won't happen at all. It seems to be more frequent on long hills but it doesn't always happen.
So here is what I am running:
Original SBC 350 bottom end. I know, ticking time bomb...
Camel hump heads
Edelbrock Performer Intake.
Summit Cam Duration 282/282, Lift .465/.465
Holly 670 Street Avenger.
New Accell Points Dizzy.
New wires.
New Coil.
New Summit 110 gph fuel pump.
Fuel lines to the carb from pump are 3/8" rubber.

I just adjust timing and valves and still no difference.

Thanks for your help!