69GMC - Lack of Secondaries/Low High-End Power

[RhapidFyre]

Just finished restoring the basics of the engine to get the whole thing back up and running. Flushed the fuel lines, replaced coolant, replaced the thermostat, got a brand new Edelbrock carburetor, set the timing, and the truck runs beautiful at idle.

First Drive Result:
Truck isn't shifting properly, fine, I'll adjust the kick-down as I see fit.
Transmission isn't shifting appropriately. Found out I forgot to fill the fluid. Works fine now. Shifts appropriately, no lagging/clanking anymore.

Second Drive Result:
Automatic (D): Hit the gas from a dead stop. A slight response lag, RPMs fell quite low, but it moves and not 2 seconds later it's taking off. Up to 40 MPH, secondaries roaring. Truck shifts to second. Initial power is good, climbing to 55. At almost exactly 55 MPH, the RPMs start to fall, it starts to chug, and struggles to maintain 55 MPH. It shifts to third gear, and all power is out the window. I'm losing speed at this point.

Manual (L1->L2): Hit the gas from a dead stop. Immediate response, truck takes off like it's a newborn. Screaming secondaries all the way up until I feel uncomfortable (around 45MPH), and then shoved it up to L2. Truck shifts immediately to second gear. Right around 60MPH, no secondary kick in. Truck's still running on low-end, holding 60MPH. No struggling, no chugging. Refuses to go any faster, and it does NOT appear the secondaries are engaging. I put it up to D, it shifts to third instantly, all power is gone. Struggles, falls down to 40 MPH and almost dies. I throw it down to L1 at 35MPH, instantly shifts to first gear, secondaries kick in, truck is at a high RPM. Roars when I hit the gas.

I'm completely at loss for what to do. I ruled out the transmission, because it's shifting immediately and appropriately. I ruled out the carb, it's brand new (750cfm). Secondaries open and roar at idle and first gear, but second and third are killing it's momentum.

Father in law believes it's the distributor, so he wants to exchange it. We have yet to do so, but I wanted to post here to farm ideas incase I am missing something.

Thank you!