Truck running poorly under load, need help.

[E.D.75GMC]

Alright first off, it's a 1975 with a 350 four barrel and TH350. Now the issue, when I'm cruising around and I need to accelerate decently (I.e merging up to a higher speed or passing), the truck starts to stumble and feels like it's about to die. Imagine a bad misfire, that's what it feels like. When you're in neutral or park you can rev it up quite a bit and it runs PERFECT. Idles just fine as well. ONLY when you get into the throttle when in gear does it feel like it's about to die, and I've noticed it's quite a bit worse when you're going up hill. To the point where even gentle acceleration feels like it's going to kill it. Within the past three years I've replaced the fuel pump, distributor cap and rotor, ignition pick up/coil, spark plug wires, and more recently (18 months ago) I've changed the spark plugs and had a rebuilt quadrajet installed. I made sure there are no vacuum leaks, and the fuel filter is clean. I had the shop that I've been taking the truck to since I bought it in Feb 2013 take a look and they couldn't find a source for it. One more thing, I took the original narrow flute opening air cleaner off and tried one that has the cold air duct from the core support but that made no difference.
I'll see if I can record a video of the issue and post it here. But in the meantime, does anyone know what may be the problem? It's been happening for months and I can barely drive the truck anymore.