surging/cutting out while driving

[SeanPaintsFlames]

I'm really hoping you guys can point me in some direction.

This 86' 305 carb/electric fuel pump truck is doing a weird thing. It has always done it in some form or fashion over the last 3 months I've had it, but it is getting considerably worse. It started when I was getting onto the highway- I punched it, it downshifted, then as soon as it shifted into third (350TH) it started to do this bucking/surging/cutting out thing. I let off the gas and it stopped.
Now it seems that either hot or cold, uphill or down hill, whenever I give it any more gas than a slight feathering, it wants to do that. It never kills the truck, but it's like someone flipping an on/off switch back and forth, or like riding a really slow mechanical bull...
It idles great, revs great in park, but in drive, especially under load, it surges. In attempt to fix it I've replaced the fuel filter connected to the pump, cleaned the like-new one in the carb, dumped half a can of SeaFoam into the carb, run a can through the tank, and disconnected all the emissions vacuum lines, plugging the unused carb ports, and removed the belt to the air pump. Nothing has seemed to make a difference, and it continually gets worse as I drive it.
If it matters, I've got a dual tank setup with only one tank in use. The other is an empty new one mounted but with no filler neck yet.